I've always prepared our tax returns myself, manually at first, then using TurboTax since the mid 90s. Same reasons others have posted... mainly to understand the process well enough to enable effective tax planning, especially during ER. I also do DMIL's return and I help each of the kids. I get TurboTax Premier for free from Fidelity, which is good for 5 free e-filings. So the kids both come over and use our PC and software. When they get stuck or have a question, I help out. I also look over the return and give them some tax planning advice for the following year.
There were a few years when I did an expat assignment for Megacorp. I had a tax equalization agreement under which Megacorp paid all my incremental tax associated with the assignment. In return, they received the benefit of my US foreign tax credit and engaged a Big-4 accounting firm to prep all returns (US and foreign). It was a VERY complicated process involving multiple hypothetical returns and lots of reconciliations. My actual US return one year was 95 pages long. I counted. There was another 100+ pages of supporting documents and reconciliations. This went on for 3 years after retirement due to option exercises that were deemed to be foreign income and thus Megacorp got some benefit from my giant FTC carryforward.
Anyway, I was a bit nervous transitioning back to TurboTax on my own after all that. But it seems to have worked out fine and I'm very glad to be preparing my own returns again.