when/where and why do I need to use the WFS-T capability of GeoServer?
I think the answer to this question of need is of course: never
If you think you have a better way of handling edits to features in your database, then do it your way. Whenever you write code for any purpose there is never a one hat fits all way of doing something, you should always consider the benefits and negatives of taking one approach over another.
Obviously you have already gone through this process to some degree on choosing GeoServer over MapServer, deegree, or ArcGIS (other WFS server software exists)... for providing you with a WFS service. PosgreSQL/PostGIS over other data storage, OpenLayers over Leaflet... PHP over other scripting languages...
With WFS-T you get a standardized way of allowing edits to features. So you might want to consider what the advantages are of using standards. Obviously you think there is some merit in standards as it appears you want to to use WFS, I mean why did you choose to do that rather than writing your own way of extracting features from PostGIS to display in your client?
So some considerations (they could be positive/negative depending on your requirements).
- If you have a WFS-T then people don't have to use your client.
- Similarly you can have more than one client, and not have to worry about maintaining different versions of code.
- If you change your mind over use of PHP/GeoServer/PostGIS in the future you wont't have to rewrite all your code