I use Pidgin as my instant messaging client because it is open source and because it supports almost all IM protocols directly or through plugins. In my case I need support for IRC, XMPP, Facebook, Slack and SIPE (Office 365). Apparently it would be possible to use Pidgin for Signal as well, but as official Signal client is open source and works quite well I’m fine with running it standalone. As I use Linux, I can rarely hope for a native client for the proprietary IM services. Plus even if there was one, I would not want to run 4-5 messaging applications when I can have one (or two). Or to have the same amount of IM web applications open just to get messages.