But the accountant only declares what you tell them. Having an accountant doesn't automatically make you immune from mistakes!
But a decent accountant will check things over and do a "sanity check" to make sure things look right, watch out for common mistakes, etc.
If I just popped the numbers given by my clients into the software, I'd say about half would be wrong.
I cross check against previous years declarations to check for anything missing, I "prompt" clients to check for other things they've not told me about, such as gift aid donations, pension contributions, re-invested dividends, etc which are very easy to forget. I also query figures they give me which just look wrong. For small business clients, I pick up on things they've forgotten, such as mileage, use of home allowance, postage, subsistence, etc, and also things I'd expect, such as PI insurance and ongoing training costs for a professional or the annual gas safety check for landlords. If total income is over £50k, I ask whether they' (or their partner) is claiming child benefit.
Putting numbers in boxes is only a tiny part of the job. It's thinking and checking about the boxes without numbers in them that is where the time and effort goes. Plus tax saving opportunities, such as change of a year end, incorporation, etc.