You would use it on individual tracks like you would use Ozone on the master track. While people like it (me included) - you can probably do all it offers with other stuff that you may already own. Let's face it, you've got by up to this point with stuff you own. The same though could be said of Ozone - is it necessary - and many people purchase and like that.
Alloy was definitely borne out of people using Ozone on individual tracks and wishing they had something slightly more dedicated to the purpose.
There's a feeling that Alloy 2 is a noticeable step up from Alloy so a fresh assessment should be made by all interested parties.
I wouldn't call it a must. It's also not a 'fail' plug-in as it does what it says on the tin and does it well.
You can try it before buying it. It's really about whether you think you've got those features covered or you think this is more convenient. Presets - the dreaded word - but they are helpful on this plug-in. The actual plug-in is iZotope quality.