I’m coming late to the party, but let me throw in my perspective.
There are databases everywhere. On your computer, in the data warehouse, in the cloud, and on your phone.
Mobile databases are very valuable for applications that want to perform very snappily on your phone. Going out to the cloud to get data can be slow and can also fail in a poor connection zone, which makes the whole app seem unresponsive.
Mobile databases usually cache data from the primary cloud database, so the application doesn’t have to go out to get the data, and they typically go out and get the data they need asynchronously, (meaning you are doing something else and don’t even notice).
Cloud and mobile databases don’t have to come from the same vendor, and they usually don’t, but it makes some people more comfortable if they do.
Mongo has its own mobile database, but unfortunately it hasn’t grown the way cloud and local Mongo has grown. To be perfectly honest, I don’t know any developers who have used it.
Realm is one of the top mobile databases in use. Mongo bought them for 39M, which looks like a deal to me - from the point of defending their weak points.
To be clear, I wouldn’t look at this as Mongo buying developer mind share. I’d look at this as Mongo acting proactively to make sure they don’t have weaknesses that competitors can exploit. As an example, now people won’t be going to their competitor, Couchbase, because Couchbase also has a good mobile offering. Mongo has one of the top two mobile databases in use - no more argument for choosing Couchbase.
This is one of the reasons why, in my opinion, once a general database like Mongo starts winning, it becomes almost impossible for others to catch up. Where Mongo’s technology has a weak point, or where a new technology comes up, Mongo has the money and resources to buy or build, and stay on top.
As long as the management team is smart and paranoid, no one can catch them. And they continue to demonstrate that they are smart and paranoid.