As eric implied they frequently do so as a health benefit to rid indigestible foods from the stomach (ie furballs) and aid healthy digestion
I did a little bit of research a few years back on cat grass when Ziggy started with pancreatitis and was frequently vomiting.
An article I read suggested
Oat grass was a healthier option for cats rather than the standard rye, and actually good for cats with IBD. For the past few years I've purchased several times from this supplier
here.
Both my cats love cat grass, but this was by far the best grass seeds I have ever bought and even friends have bought upon my recommendation. Unfortunately they are out of stock right now.. which is a shame as I was going to get more of it just last weekend.
If it's not back in stock soon I may try
this one. The pack size is slightly smaller but because I've had some rubbish results in the past with other suppliers I'm hesitant of buying a large pack just yet. Don't buy Trixie - its carp and a waste of money.