Why do we use use DaaS
A DBaaS such as MongoDB Atlas is beneficial to the longterm growth of an application.  Running a database as a localhost server provides no redundancy and no capability of enduring failure.  


A localhost database will eventually run out of local disk space, a
DBaaS should allow you to expand disk on the fly.
A localhost database will never simply notify you of issues, a DBaaS
should contact you about any pre-defined problems.
A localhost database can't back itself up, a DBaaS should provide
database backup options.
A localhost database will not know how to handle a large influx of
new reads, a DBaaS will allow you to dynamically scale your DB while
allowing your front end app and cache to handle incoming new traffic.


Network speeds in both traditional datacenter and "cloud" solutions such as Amazon Web Services can leverage low latency networks.  These low latency networks have the capabilities of providing high performance data access speeds to keep up with the most frequently accessed documents in a database such as MongoDB.

There's never a reason to install a database in production and leave it alone on your prod server.  Your database will likely not use the same best practices as you should configure when in production.  Based on these very basic examples, you can see why so many companies do not believe in using a standalone DB on localhost, they put trust in DBaaS services.