Looking to history earlier versions (e.g., Garmin Edge 305 and Edge 705) came with barometers for elevation. This was long before Strava, and during a time when you did all the analyses on your own computer. Some analysis software supported getting elevation data from other sources, others didn't. And at the time accurate and free elevation maps were not as ubiquitous. In terms of marketing, when you release the next generation of devices (e.g., Edge 800, 810, 1000 etc) it may not look good removing features as people love to compare specs without thinking about why they may need a spec.
Some Real World Uses
When I used to race, I found getting real time road grades helpful. One example that stands out was a long false flat on one course (a false flat, looks flat but has a subtle elevation gain). Physiologically it was demoralizing. I found being able to confirm I was on a false flat by looking at the grade output helped greatly as it gave me the confidence to attack hard even though I felt slow. The attack proved instrumental, securing a solo breakaway win.
In touring I loved being able to track my total elevation gain for the day. It helps to put a slow day in perspective (if you are trying to cover large distances), or tuned me into cutting a day short due to the volume of climbing. Often you don't have access to the internet camping so real-time elevation can be useful here too.
Apart from these reasons there is no one overwhelming reason (IMHO) to insist on having a barometer with a GPS computer, but that said it can be nice to have if it is there.