The IM format is meant to be more conversational. Do you tag and categorize the conversations you have with people IRL? Probably (and hopefully) not. That’s the sort of comms that IM is meant for.
Both have their place, IMO. And it’s not like if you can only choose one or the other, anyway.
For me, I just don’t like having my genitals out when other people are around. It’s not about whether they’re looking or not, it just feels uncomfortable being exposed. Same reason I don’t use public showers at gyms and such.