Yup, data sites give you data cores that are mandatory for invention (ie turning T1 bpc into T2 bpc), even though data cores are usually rather obtained from R&D agents (requires omega-restricted science skills) or, says my first link, from factional warfare LP stores.
More often, I’ve found that data sites provide decryptors, that help manipulate invention in different ways (see invention link).
The 3 social skillbooks not seeded on the market (ie sold by NPCs) — distribution, mining and security connections — are found there too. I can’t remember for the life of me whether they come from relic or data sites, though; they’re pretty rare.
I believe there are some science skillbooks in data sites too (Takmahl / Talocan / Yan Jung technologies at least) and, iirc (that’s a big “if”), those are needed only for tech 3 research and/or production… or is it sleeper tech/encryption methods? Bottom line is: don’t bother to buy & train those 5 unless you’re sure you need them. That’s a lesson I learned the hard way
If we’re talking about exploration in the general sense, then we also have to consider ore anoms, gas sites (another branch of mining), wormholes (I can’t tell you much about those, haven’t tried yet), and combat sites & escalation (anoms and signatures, the later has to be scanned down).
In combat sites and escalations, you will loot (pirate) faction equipment and (sometimes) BPCs (eg that), that give better bonuses / at lower trade off, or can be turned into good isk. I usually see these expensive modules as “the edge” that PVP players look for.
So, to sum it all up, exploration is a low-cost-of-entry activity to find different kinds of stuff that are needed for (usually) high-cost-of-entry activities (eg T2 industry). It was designed to provide something profitable enough to do for “younger” pilots. “Selling to some one who knows what to do with it” is exactly the point!