The testicles are what produce testosterone, so removing them means no longer needing an anti-androgen, such as spironolactone. All anti-androgens have various side-effects. Also means that if you're ever forced off of HRT for whatever reason, there's no danger of being forcibly detransitioned by your own body (at least not nearly as much). Obviously this has it's own draw backs as not having any sex hormone is generally a bad thing (such as osteoporosis), but a lot of trans people would rather take those risks than deal with a sex hormone that actively does them physical and mental harm. As for breaking up the surgeries if you're going for both anyway, I think it's either a cost thing or reducing the recovery time? I'm not entirely sure to be honest. I do know that the recovery time for an orchi is considerablly less than for full on SRS.