As
@LucidSDreamr gave a good description. People choose ROA's differently most often in an attempt to mess with a drug's bioavailability. On this spectrum, oral usage is at one end and intravenous injection on the other. We're not gonna talk about Transdermal or Intrathecal administration as they're not relevant to what we're discussing.
Rectal administration is a middle ground between these two points on the spectrum. This ROA allows users a faster onset and an often higher bioavailability. Possibly the most important factor to be aware of though would be safety. Rectal administration is essentially just as safe as oral administration. I'm a major proponent of its usage and spreading the word, as I feel it's great if we can intercept people on their way to injection and give them a compromise.
A lot of people are really apprehensive of the process. There's homophobia, hygiene questions and all kinds of stuff in between. The reality is that it's a very simple thing to do. I've never had a solution come back out or even really leak at all. I know a lot of people are afraid of this. It doesn't really seem to happen in practice unless you have some kind of previous issue regarding your rectum.
You gotta love talking about rectal administration. It's so glamorous.