Not sure about the support in mail clients (I mostly use mu4e with mbsync, I don't think they can look those up), and they certainly aren't required. But I set them anyway, along those for XMPP (where they should be set), and it's potentially useful. Similarly to setting TLSA, SSHFP, CAA records: those are rarely used, but they do seem nice to me, and it shouldn't harm to provide clients with such an option. Also the more servers provide it, the more motivation there is for clients to support it.

why do you need to develop SRV record support in email clients? are there email providers who publish SRV records?

it's like why do servers/clients need to support IPv6 if there are few clients/servers doing it :)