So I have a MIJ Fender Jazz, and I was wondering if replacing the bridge does anything except make the bass look cool ?
Does it affect the "ease of fretting strings" at all ?
Does it affect the sound/tone/intonation ?
Aftermarket bridges usually are higher mass which adds greater sustain and frequently are more stable. Not all types of music require milking all the sustain you can out of the instrument. The screw threaded Fender Vintage Bass bridge is one of my favorites. The Leo Quan aftermarket high mass ones are very well designed but unless you get the ones with the pregrooved saddles they are going to be a pro install...meaning you have to find someone with the right files to slot the bridge so figure on $50 more for the bridge setup.
Bridge mods are no different from any other mod, they often turn out to be a can of worms. I too use both. In my opinion what you get with a high mass bridge is not a better tone, but the same tone for longer. Unless you have a bass that won't intonate with a stock bridge there is no intonation advantage. Sometimes the height adjustment screws on the Fender bridge can move on their own, but some clear fingernail polish stops that. A high mass bridge won't make your bass play better, unless your existing bridge has some issue or you just wind up with a better setup.
On a passive bass there are other things to spend money on that make more tonal difference than a high mass bridge.