I really don't get that penis-bottle-thing he is using. Couldn't he just use his hand?

That's an onahole and it's designed to simulate a vagina.

Well, no. Using a sex toy gIves a totally different feeling. It can touch more skin at once, reach places your fingers can't, has a different texture... sure, masterbation can be simpler, but well... the feeling that it isn't your own fingers is a feature.
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