Alongside the mechanical watches on the bench, I keep a quartz Seiko in the drawer, and I wear it more often than most visitors would guess.
A plain case for accuracy
A quartz movement will hold time to within a few seconds a month without any attention. That is not something a mechanical watch, however well serviced, can match, and there are days when knowing the exact time matters more than the object telling it. It also runs through the days a mechanical piece sits on a timer or waits on a part, so there is always something on the wrist.
None of this makes the mechanical pieces less worth owning. The appeal of a hand wound or automatic movement was never really about accuracy, it is about the visible mechanism and the maintenance it asks for. Keeping a plain quartz watch in the drawer is simply an acknowledgement that the two things serve different purposes, and that a collection is more useful when it is not making the same argument twice.