"Need" is that it is something that makes the app useful (as I don't want people receiving to know I am not in the Office). As my desktop client send in HTML format and I tend to use formatting to emphasise meanings, when they get a plain text reply is will immediately tell them "not in office". It is a bit f a "fraud" in some respects as small organisations like to pretend they are larger than they really are and thus have a "team" always available for support in the office (ready and waiting for their e-mail). However, in the real world I am out and about visiting customers but every client like to think they are special and have 100% of our time ...
Plus, as I use formatting to emphasise, it is something I would lose. I tried the *bold*, etc. and my e-mail client displays them as raw text (i.e. *bold* appears as an asterisk followed by the "bold" characters then another asterisk.
So "need" could be rephrased as it being a very important feature.
Edit note:
For my own needs, whilst a full wysiwyg editor would be great, were the app to convert the *bold* /italic/ etc. into an html message with the formatting it would be great (i.e. it created an html message but no need to develop wysiwyg makes development quicker and easier - though I'm sure longer term would mean full thing).