Well, but 25 astral pearls gets you the search-power of about 20 specific gems (2 of each), all packed into one immediate result for one turn by one astral mage. Otherwise, you could waste 10-20 or more mage-turns having various mages cast or physically move to and search the province, and it would require a great variety of magic powers to do so, rather than just one path, and you still might not find all the sites, and maybe not get around to it for a long time. Not only is that a big savings in mage-time and skill requirements, but the earlier you find a site, the sooner you can use it, and put its uses into effect, which can beget other effects, etc.
Of course, especially with a lowish site frequency, you could also blow 25 pearls and find nothing you want.
As for the "what do do with astrals" question... give them to me!
Astrals are the gem type with the most uses - if you don't have anything in mind to do with them immediately, then either save them for later (like when you get a really good reason to cast a strong Dispel spell), and/or alchemize some of them into gems you don't have yet, and/or gold you need.
PvK