I used to just go and buy ... then one time I saw a lady with FQs in a box, pattern in hand ... and scoping out the fabric at a store, for that pattern.
From then on, I thought, this makes sense!! So now, I start a list in advance and know what I'm looking for. It may be all one project? or it may be odds and ends of supplies that I want/need and haven't found?
Of course, the shops want you to purchase, though they know that realistically not everyone will. Please don't feel that you MUST!!
This summer for the Ontario Yours to Discover, I went with a mission of the 4B's ... Borders, Bindings, Backings and Battings ... as an education goal. It also was a great discussion point when I was at the shops and I saw and learned things I wouldn't have, if I had not mentioned it to them.
And yes ........ I do make those spontaneous purchases!! Though I have found that I am doing less of that, now that I have a 'mission' planned before I head out.