Not all of the following need or should apply in any particular instance.
Historically, this must surely have been the main reason. A god is worshiped because he or she is powerful; not to worship might cause punishment, to worship may be rewarded.
A powerful god need not be good, but might indeed be evil. So, worship of power is not 'good' but rather expedient.
This is much the same psychological mechanism as applies to attitudes to a Chief, King or Emperor.
A god may be reverenced because he is Father or she is Mother - either personally and/or to the group, the nation. God is our creator; such a god made us Men.
The reverence of a parent is natural, spontaneous; and that this reverence is a good thing has been implanted into us (although it can, of course, be over-ridden by other factors.)
Love between parent and child can be regarded as good if we assume that its goodness comes from the goodness of god - otherwise it is just a contingent by-product of natural selection.
3. Creator of all things
A god may be revered because he created not only us as persons, but also the world we inhabit.
Such reverence is based on gratitude - what is has been made, was made by God and 'given' to us.
The concept of creation implies that god made things as they are, made them into what they are - it does not imply they were made from nothing. Indeed, creation is spontaneously understood as an act of shaping and forming pre-existent 'stuff'.
4. By rational necessity
This is the philosophical doctrine that god is necessary, and that therefore (by a chain of reasoning) it is necessary to worship god.
Not to worship god is regarded as a product of ignorance or confusion. Not to worship god is regarded as not-so-much evil as illogical.
This is associated with the omni concept of God who can do anything rational and is outside time and space: omnipotent, omniscient, omnipresent; and who created everything from nothing.
Personally, I revere God because of 1, 2 and 3 - but not 4.
The point is that there are still at least three reasons to revere/ worship, be grateful to a god - even when the validity of an omni-god is denied.