I would say convention is on a square or rectangular parts or a part where all the pins are along the outside they are numbered.
Anything that is in an array with pads underneath the part is typically in an alphanumeric grid. For example bga, PGA, lga, etc.
Benefits? Well most people do it that way :). This will most likely march the datasheet. For a rectangular part I expect to be able to count around the outside of the part to find my place when debugging.
For bga where you could have 2000 pins it would get a little messy with the large numbers. Plus it's easy to find your way around in review or debug (or when designing your pin map if you've made the asic). A2 is right above b2, but if it was numerical you'd have to count or do some math in your head to find the next pin below. Again this will likely match most datasheets keeping in mind they often skip ambiguous letters like O.
Just some thoughts hope it's helpful.