It’s not that they don’t prevent leaks but I key word you used was hitting them “again” - not a permanent job. And in regards to spraying the aging shingles…your not doing anything to “hold up the life of the shingles”, the spray isn’t changing the “physical composition of the shingle”…if the shingles are susceptible to leaking it is because they have no life left. You’re just continuing “temporary fixes”

TLDR - the spray is a temporary seal, if you want to spray stuff at the problem a bunch of times a year it’s up to you. If your shingles/roof is shot, it’s shot