Hi! I have never heard of it being a necessity - and none of my cats have ever been given it. But, Feeby will try to do anything she possibly can to get to any spider plants we have in the house!
Some cats like to eat grass. It's like a treat to them. Some people feel that grass helps with indigestion or hairballs. It's not necessary at all to feed your cat grass. If you want to give it a try, pet stores often sell small pots of grass. Avoid the dried out looking ones. Show your cat the pot and pull out pieces of grass if your cat has no clue what to do. Offer it like a treat. Your cat may be curious and sniff the grass and may eat it. Or your cat may ignore the grass. Water the pot every other day or so. When it starts to get rangy looking and yellow, toss it. If your cat just ignores the pot of grass, no point in buying more.
One of my cats goes nuts for grass to the point where he eats too much too fast and it all up My other cat ignores grass. I buy cat grass seeds and grow them in soil.
Catnip is another safe plant you can grow at home. Pet stores and garden centers sell small pots or just grow them yourself with catnip seed and potting mix that doesn't contain fertilizer. You could grow catnip directly in the ground if you have a yard but then you risk attracting strays and ferals.
Several years ago, I bought a small pot of grass at Petsmart on a whim just because it was next to the cash register. Some of our cats loved it, and some didn't care for it at all.
But after buying another pot or two for about five bucks each, I bought some seeds from Amazon and started growing my own grass for a fraction of the cost. I bought several containers to use as pots (actually, I think they were supposed to be candle holders), and "rotate" the crops so that the freshest are always in the areas of the house where our cats spend the most time, and move the not so fresh containers of grass to the areas they rarely go. Then once the grass starts looking pretty puny, I toss it and plant some more seeds, and move the containers again.
Much cheaper to buy the seeds yourself as cheeser
suggests. It is not that expensive to buy organic seeds and soil for that matter as you use very little of the seeds in order to get a result. I have used cat grass inside for years and most of mine look for it.