It depends on the circumstances, there can be big gap between an offer coming in and proper legal work starting, someone can make an offer, but not yet have a mortgage offer, which might prove impossible.
If you have an older house, you may want to keep your options open until the building survey has been completed and you know whether the buyer is going to use it as an excuse for making a much lower offer, that you decline.
You may want the estate agent to do a thorough check on the potential buyers, claims, about their house ebing under offer, how long there chain is, how secure it is.
I can thing of an hundred reasons, you might want to keep your property open to viewings after and offer has been received and accepted, although I do agree once the survey is done, mortage approved, and the chain tested and the solicitors get down to work. Yes, after that, viewings should cease.