Here is the most logical explanation I can give you. By the way, super VALID question. So say you have all those blades enabled, but no ssl inspection. Yes, it will do its job, but you will miss out, because without ssl inspection, all you will really be able to tell if that say someone went to facebook, but you have no idea what they did while there.
With ssl inspection, you can get all of that data.
HTTPS Inspection means GW is terminating TLS connections, decrypts them on a client side and re-encrypt to the server. This allows inspecting the data flow, applications behaviour, and content sent through TLS tunnel with multiple blades: IPS, AVI, AB, etc.
Plus, considering that probably 99% of websites nowdays are https (just my educated guess), it only makes sense to have https inspection enabled. Also, with R81.20 version, I find it works very well, no issues.
Hope that helps.
Andy