Most problems related to scalability appear out of nowhere without giving advance notice. If you did not design your application to scale, once these kinds of problems appear, you will probably need to invest a lot of time just to keep your system responsive.

You can decide to scale your database for a variety of reasons, such as to support a higher volume of users or provide a better performance experience for them, store a larger volume of data, and improve the current system availability through geographic dispersion.

Obviously, there is a high cost to scale your database that you should consider, such as cost of hardware (adding or upgrading of servers) and cost with maintenance.