> Another benefit, which may or may not matter to you in particular, is that the client can handle multiple email accounts without mixing them. This generally isn't possible in a webpage unless there's a single service that has access to all mail accounts, which isn't desirable for privacy & security reasons. But this is admittedly a niche feature.

Also nice is that you can tile the original email and the response side by side for sake of context, rather than having to scroll excessively.