lawmakers

The phrasal verb "allow for" means "include" (in a calculation or estimation). It can also mean to "make possible".

It would have been better if the writer had not split the phrasal verb and had instead written "adequately allow for".

Your quoted text may be rewritten as follows:

When lawmakers establish income tax rates in order to generate a certain level of revenue, they do not adequately include in their calculations revenue that will be lost through evasion.

You should allow for delays when planning a journey.

You'd better leave early to allow for traffic jams

When you make your financial plans, don't forget to allow for inflation.

When planting a tree, choose a place with leave plenty of space to allow for root growth.

excellent explanation

it was very much helpful

thanks

I made an error - I should have written this

When planting a tree, choose a place with plenty of space to allow for root growth.