One of the HUGE benefits of e-file for them is that there are sanity checks that happen almost immediately and if they fail the IRS completely rejects the return. This does two things:
1. ensures accepted returns look much closer to accurate
2. places the burden of rejection notification on the e-file transmitter. For example, if I transmit a tax return that the IRS rejects due to a re-used SSN, the onus is now on me to notify that filer that their SSN has likely been stolen and provide customer service on what they should do next. From the IRS perspective this is a huge win. For e-file transmitters and tax software providers this is an un-fun problem to deal with.