It would be unusual for one of "us" to get a different figure at some tax prep place if the inputs are the same. It isn't magic, and they use software much like TT et al.
The main thing I like is how TT downloads my info from Schwab automagically.
as a tax/accounting professional, the benefits to having an accountant isn't necessarily them knowing which forms to fill out. as has been said, the tax programs aren't dissimilar from tt and other do it yourself software. if all you have is a w-2, some int/dividends and/or stock sales, mortgage interest and other itemized deductions (likely not going to matter for most anyway, for fed), student loans maybe, then you'll probably be fine in the do it yourself category. and this is generally what i recommend to friends and clients when they ask.
where it's really beneficial is when you have business(es) and/or other unusual tax situations and can do some tax planning. and this is usually done before year end. if you wait until after year is over, there's little that can be done, it just is what it is.
Then they shouldn't be charging people anything.
most of the time, people doing the data entry for the big box tax prep places have no more experience and expertise than you. they aren't cpas, aren't accountants, and usually aren't even registered agents with irs. it's a seasonal $8-10/hr job for them. and then an accountant/cpa/registered agent comes in behind them and reviews the forms for errors. i'm sure most do a decent job reviewing them, but i've seen more than a few that were... less than satisfactory. a lot of times, they also have so many to review it's nearly impossible for them to give good oversight to all of them.
as for not charging people for it, well, as i said above, when asked about it from friends/clients, i'm honest with them on how they might save some money by doing it themselves. and they'll likely be ok. but i have no issues doing a simple return if that's what they choose. and my time is valuable.
