In order for SQL Server to read and update data, the data must be in the buffer. SQL Server does not work directly with the data in the files on disk. Once the pages of data are in the buffer, they can be used for multiple queries. This means that the data doesn’t have to be retrieved from disk every time it’s needed, thereby decreasing the amount of I/O work required.
You may have seen this yourself when selecting all the rows of a large table twice. The second time, the query can run much faster because the data does not have to be copied from the disk to the buffer. If you run another query from a different large table, it may cause the pages from the first table to be removed to make room. If there is not enough memory, pages will have to be read from disk more frequently causing your queries to be slow.
Even with extremely fast SSDs and flash storage arrays, RAM is still typically an order of magnitude faster, so having enough RAM and using it wisely is critical to a well-functioning SQL Server instance.