I used to enjoy doing that on a grey Saturday afternoon, but most charity shops in my area are now much too savvy to leave any treasures in their shops. A volunteer checks any 'interesting' books for value online and sends them up the line to HQ to sell on eBay or Abebooks.
The remaining dross goes onto the shelves for 'five books for a pound'.
It's profitable for them now but in the long term I think itis counterproductive since it removes a lure that used to encourage people to enter the shops.
Sometimes I buy something, sometimes I don't. But it's always fun looking through the titles for treasures.