Kendra Little shows how read-ahead works, using the example of index seeks:
Looking at an actual execution plan, I dig into the index seek operator and it shows me information about the physical IO. Almost all of the requests were read-ahead reads.
Read-ahead is a mechanism that SQL Server can use when it’s pulling a lot of information from disk. Instead of pulling 8K pages onesy-twosy-threesy, SQL Server can suck up big chunks of pages from disk with a vacuum cleaner.
If you’re running developer or enterprise edition, you may get a larger vacuum cleaner.
Read-ahead is a good piece of functionality, but those reads still have a cost associated, and the cheapest read is the read you don’t do.