I want to check my mail. For that, I can open Google Inbox or GMail in a browser, or open my desktop mail application (Evolution from GNOME's suite).
When I open Inbox, I can instantly say good bye to almost 1GB RAM, depending on the browser I use. If I stay with GMail, it's only cca. 600MB, but still a lot. And I have two accounts I need to check regularly. Even my notebook with 8GB RAM can struggle under them.
Or, I can start Evolution. At its worst, Evolution eats up about 200MB of RAM, even if both accounts are configured.
Quite a difference, isn't it? Slacks Electron app is similar: if I have 2-3 accounts configured, it can eat up a GB or more. If I configure my accounts in a regular (although unofficial, thus, not supported) desktop client, I can stay well under 200MB again.
Until this problem is solved in browsers, desktop apps will remain a thing.