Why do some players have cash and chips on the table when playing poker?

Answer 1

That's because of the "table stakes" rule.

Money on the table can be played in a hand, you just can't go to your wallet during the middle of a hand.

Occasionally, a player will sell another player chips for convenience, but most house rules prohibit taking money/chips off the table, so if for example, I'm doing well and have big stacks of chips in front of me and I'm near the end of my session, I can sell you $100 worth of chips, but I have to keep the $100 on the table.

Answer 2

If you are referring to TV poker, they do it for show.

Normally most houses have something called "table stakes", which means you can bet up to whatever you have on the table at the start of the hand. If those limits don't apply, the gamblers may just pull a wad of cash out and use it instead of taking the time to convert to chips. Once on the table they leave it for gamesmanship and to make the opponent think hard before pushing in a call bet.

It really depends on the house rules.