Perhaps you started out with X amount of memory when your SQL server was brought online and over time, with additional load and activity on that SQL server, users are not quite getting the type of performance they’re used to getting. Sure, you can buy more memory. What if that’s not an option?
If you’re running low on memory and need a little boost, enabling buffer pool extension can take advantage of an SSD as an “extension” for the buffer pool.
This is one of those interesting features that probably help a small number of customers but shouldn’t be generally useful. That’s because even with SSD performance improvements, memory is still a couple orders of magnitude faster, so as long as you have the ability to increase RAM, that brings much better performance.