Firebird may be configured to use large own page buffers cache. In such case filesystem cache will duplicate Firebird's one and give no or small benefit on file reads wasting memory for double caching. It still helps on file writes but only if Forced Writes setting is OFF which is not recommended in most cases.
Proposition is to allow Firebird to not use filesystem cache to free more memory for another operations such as sorting and\or caching of another database etc
Often, when Windows tries to cache a big file, it moves the FB page cache to the swap, thus causing *a lot* of unnecessary paging. In my practice, if the FB page cache size is set to more than 80% of the available RAM, you're getting big troubles. Of course, I speak about SuperServer only (the OS file cache is *a must* for Classic).