From my side, I can provide you some "storage-agnostic" answers to why people chose Veeam over SAN snapshot based technologies (I assume Compellent is not so much different):
Recovery
1. Most of the existing technologies require installing agent in each VM to enable granular recoveries, and people hate dealing with agents, especially when they have hundreds of VMs to babysit them on. Veeam does everything without agents.
2. The recovery software is normally built for physical systems, so it normally does not VM recoveries very well, because it treats VMs as physical servers.
Backup
As you already mentioned, snapshots aren't really backups, so you do need to look for implementing real backup to 3rd storage. What are your options?
1. Buy another similar storage box and use SAN based replication. This is an extremely expensive option, and not a good solution from technical perspective. We've seen it too many times and with many storage vendors when SAN based replication would replicate corruption to DR storage (our best success stories come from disasters like that). True backups must be read-only, NOT participating in any sort replication/synchronization process with the production storage.
2. NDMP backup to tape addresses all of these concerns, but makes the recovery time unacceptable. Time to recover a failed VM with Veeam is less than 2 minutes, and I will let you imagine yourself how much time would it take to perform such restore from a tape backup.
Veeam enables you to create disk-based backups of your production data, thus enabling you to perform recovery on any level instantly, but we are 100% backup storage agnostic. For example, Windows or Linux server with JBOD makes an awesome backup repository for Veeam, and I will let you compare the price of that with another Compellent SAN. So, going with Veeam enables you to save a lot of money in both CapEx (much lower backup storage cost) and OpEx (faster, guaranteed recoveries from true backups).
Now, being a very mature product we also have tons of virtualization specific technologies those SAN based tools cannot even touch around both backup and recovery. For example, automated backup recoverability testing (when was the last time you tested full VM recovery of your entire environment, something you will need tomorrow if your SAN RAID controllers decides to corrupt all production LUNs). Or, self-service file and VM recovery through the web-portal. Check out all the major features our product provides
here.
Hope this helps!