As university faculty, I write letters, 100s and 1000s for UG students, G students, and other faculty. I can usually tell if the letter will matter by the person and position applied for before I start. Maybe I am wrong sometimes, but not often.
Some occasions are pretty boilerplate. The request is only formal, a box to check. Other times the applicant is a good match to the position. So the letter just has to be accurate. Sometimes, however, letters really can make a difference if they are eloquent and revealing: Occasions when the competition is tough or there are hidden strengths or visible weaknesses to address. A good student letter, one that counts, can take an hour or more to write. I can see why HS teachers have to limit the number. I always will write for a student, but do limit tenure letters as they can take days.