Well, for one, they're useful for sealing in moisture. Oils can be used similarly to cones to protect and coat the hair shaft from damage, as well as moisturizing. Often times, people here choose to use them instead of cones because they're easier to cleanse out without SLS.
Oils that penetrate can 'strengthen' (I assume, never actualy heard anything about this being proven xD) and often prevent protein loss (at least coconut can), but the other ones ARE still useful as well.

The ones that penetrate just do a little extra, I think.
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