بسم الله الرحمن الرحيم
Importance of Tawheed

The natural inclination towards the affirmation of tawheed is part of the fitrah, in which Allāh subhānahu wata a'lā created mankind.
 
Tawheed means to believe that Allāh is the one and only true God, and He alone deserves to be worshipped.  Tawheed is sometimes referred to as monotheism.  However, in its purest sense in the Islamic definition is that no other entity can in any way have Allāh's perfect attributes or be equivalent or even in competition with Him.  All acts of worship that a person does should be directed to Allah or for Allah Alone.  For example, prayer should only be directed to Allāh, religious animal sacrifice should only be done in Allāh’s name, or fighting to establish and protect religion (Jihād) should be done for His sake.  Tawheed is therefore the central concept upon which all of Islām rests.  It is the core belief ('aqeedah) around which everything revolves.  Indeed, it is the Islāmic position regarding the belief in Allāh that makes it unique among the world's religions.  Other religions claim to worship one God (monotheism), such as Christianity and Judaism, yet unlike the Islāmic concept of God, they either attribute to men what befits only God or they attribute various weaknesses, faults and failings to Him that do not befit His Majesty and are instead more suited to His creation.
 
In the Islāmic concept of Allāh, none of the weaknesses, failings or needs of the creation can be applied to Him.  He is indeed not of a particular race nor does He favor any particular race among His creation except those who are most devoted to Him.  Muslims believe in Allāh as He or His Prophet have described in the Qur’ān or the authentic hadeeth – that is, the authenticated statements of the Prophet Muhammad himself (sallallāhu ‘alaihi wa sallam).  His primary function was to teach mankind about Allāh and correct their wrong ideas.  People have no right to interject or interpret what they have no ability to comprehend on their own.  Whatever people have made up regarding Allāh without any basis from the final revelation of the Qur’ān and sunnah is purely conjecture from their minds.  The sunnah is literally the manner and way of the Prophet Muhammad (sallallāhu ‘alaihi wa sallam).  The reports of his speech, actions and tacit approvals are called hadeeth and from these reports scholars derive the sunnah.  The terms hadeeth and sunnah are sometimes used synonymously.
 
Importance & Benefits of Tawheed
 
Tawheed is the call of all the prophets to their people.  Allāh subhānahu wata a'lā states in the Qur’ān:
    And verily We have sent among every nation a Messenger proclaiming Worship Allah Alone and avoid all false deities. (soorah an-Nahl 16:36)
 
Fulfilling tawheed is the purpose of man's life.  Allāh subhānahu wata a'lā states in the Qur’ān:
    And I created not the Jinns and Men except that they should worship Me. (Soorah Adh-Dhāriyāt 51:56)
The word Jinn here refers to a separate type of creation that live and die, have families, believe and disbelieve like mankind but who are fundamentally of a different physical and psychological nature.  
 
By declaring sincere belief in tawheed, a disbeliever will become a Muslim.  Conversely, a Muslim will commit disbelief (kufr) if he rejects any aspect of it.  By tawheed, we achieve salvation from eternal punishment in the Hereafter, right guidance in this world, and forgiveness for sins.  Allāh states in the Qur’ān:
    Verily Allah does not forgive associating partners with Him (shirk) however He forgives all other sins as He pleases.’ (Soorah An-Nisā 4:48).
 
Prophet Muhammad sallallāhu ‘alaihi wa sallam) said: The worshipper’s right on Allah is that He will not punish those who worship none besides Him [Al-Bukhari and Muslim].  Whoever fulfills tawheed will be admitted to the Garden (Al-Jannah) in the Hereafter.  Prophet Muhammad (sallallāhu ‘alaihi wa sallam) has stated: Verily Allah has promised that whosoever does not commit shirk shall enter Al-Jannah (the Garden in the Heaven). [Al-Bukhari)]
 
It is the way to relieve people's sadness because true happiness can be achieved if people realize tawheed within their hearts.  They will also not feel psychologically enslaved even if physically captive.  It is the only reason for Allah’s pleasure and rewards.  In another āyah in the Qur’ān Allāh says:
    If you reject faith then know that Allah is in no need of you and He does not accept disbelief from His slaves.  (soorah az-Zumar 39:7)
 
Disbelief is nothing but a manifestation of shirk which is the opposite of tawheed.  That is, one who disbelieves inevitably breaches some matter of tawheed.  The acceptance of all of one’s religious deeds is based upon tawheed.  Allāh 'Azza wa Jall states:
    Whoever accepts a religion other than Islam it shall be rejected by Allah and in the Hereafter he will be among the losers. (Soorah Aali-'Imrān Āyah 85).
 
Islām is indeed the embodiment of the belief in tawheed.  Tawheed prevents man from eternally remaining in the Hellfire.  Prophet Muhammad (sallallahu ‘alaihi wa sallam.) stated in an authentic report: Whoever dies and has so much as a mustard seed of faith in his heart shall enter Al-Jannah.  Faith here signifies a correct belief in Allah and His Messenger Muhammad (sallallahu ‘alaihi wa sallam) and all that they instructed, commanded and prohibited for mankind.