Most Players are interested in trying new towers and rent or buy towers from the market but forget to upgrade towers is a something that you have to consider as upgraded common tower may be better than epic one and if you have upgraded epic tower then you may have a powerful weapon that you don't have before and you would do better in the battle.
For Example when you start the game and you have the tutorial then you added a Gloom Owl Tower and then you have seen its effect on fiends and when you asked to upgrade it to get a higher damage, I have remember this tower become so amazing and have the ability to eliminate most fiends that two normal towers couldn't.
So when you upgarde towers for damage you benefit from 100% increase in tower damage and you could kill fiends quickly whether one fiend to be killed or a group of them could be eliminated quickly if you use a tower with tesla or area attack and you may also use a tower with chain attack and benefit from its high damage.
There are also upgrades other than those of damge and also lots of us don't try it as you know you could upgrade tower for range attack and increase its range in the battle and in this case you could benefit from its damage or speed in a wide range and this means attacking fiends for a long period of time.
Also You could upgrade its speed and this would help with towers that have a high fire interval and so you could get a better fire rate and a tower that you couldn't use as it is slow and fiends could pass from it quickly then you could use it and benefit from its features.
