It's definitely for showoff, but also some do it to attract or influence other people into gambling sites; they are giving the wrong idea to the viewers on how easy it is to earn in gambling sites such as that, but in reality it is not. In short, they usually do it to attract more players and deposit in that site, as well as for referrals. In my opinion, keeping a huge amount of money on a gambling site is not a good idea. There are many reasons why, but mainly that your money could be gone.
Keeping money in the gambling site was a stpid idea but many people are still doing it right now. I meant earning more money through referral was good but it also depends on how active our referral is. So many time keeping our money in the gambling site be result in a lose once the site will be gone or disappeared due to the hack.,
It's quietly common for influencers or high rollers to this to show off their balance and attract users on the platform with their referral link being posted along the screenshot of these incidents. They might not be active the amount of reach these people gets earns enough through referral. There are even instances that these people are even related to the gambling platform itself especially new ones so they know the status of the platform before they withdraw they funds but that would be a separate topic from what the OP raised.
I too keep some funds or winnings on the same gambling platform I play on so that when I'll be playing again, I won't have deposit and instantly play however if the amount is too significant, I would rather withdraw it than keeping it for long on my account as I've personally experienced my funds being held by the gambling platform but was able to withdraw with some documentation and back to back discussion with the support.