The only reason I can think of is to make sure the bores are correct.
The other thing is, especially on production blocks, you could get some that had an oversize lifter bore. They are rare, but they did happen. An oversized lifter or two was cheaper than scrapping a block.
The other thing is along those same lines...the bores get too loose you that is the only way to correct them.
When installing dorf lifters in a Chevy I almost never bushed them. As long as they cleaned up, we just corrected the bank angles and all that and let them run.