The benefits of being anal only, even for only few days, are that one's sexuality is a lot easier to manage - before I was fapping a lot and I had trouble stopping (yeah, fapping / sex can become kind of an addiction, but that's a separate matter). I found out that being anal only makes one a lot more aroused for the entire day but this kind of arousal is entirely different from "normal" arousal felt before using penis - it doesn't distract that much and is overall pleasurable. Also, though male anal orgasm is also just an ejaculation, it doesn't make one's mood so low after the thing's over - I strongly believe that many of us, who are interested in anal only lifestyle, may suffer from post-coital tristesse.
Another interesting thing related to anal orgasms is that they don't originate in rectum (although the pleasure is obviously felt there) but rather in bladder. I speak from personal experience which is consistent with the reports I read on this forum. I think that anal orgasms originate in urethra and bladder sphincter.
My advice for males wishing to achieve anal orgasms is to be aroused, to find pleasure in the "uncomfortable" feeling when one wants to pee and it feels like he can't take it anymore and push - I'm saying about pushing like for peeing, not bowel movement.
Another important thing is, in my opinion, that we have to accept that we are wired the way we are and our sexuality revolves around rectum / bladder. For very long time I couldn't accept that I am wired that way and this is what caused me to fap a lot more that I would like to - my body wanted the satisfaction that anal gives but without anal play I couldn't satiate myself. Now that I fully accept my sexuality I want to try month of anal only to change my habbits, experience a constant, significant improvement in my mood and never again experience the low mood after penile orgasm. The only thing that worries me is that after a week I may get too horny, the state I don't like, but I suppose that after some time the horniness level will normalize.