I have caught my cat a couple of times chewing on household plants. Most of the plants now have chewed leaves. I have seen him do it outside but I suspect he might.
Why does my cat chew on plant leaves?
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Roughage, to aid digestion, help them pass/regurgitate hairballs, and sometimes just because they like the texture or it cleans their teeth (same reasons some cats like puncturing paper).
If you want to discourage nibbling on your houseplants, consider planting a pot of cat grass (in the literal sense, not "kitty pot" aka catnip). Most pet suppliers offer seeds for one or another member of the grass family, for just this purpose.