What do you think the reason is? I had hypothesized that it was because I keep tons of tabs open, and over time more of them are made active (they are in a quasi-hibernated state when I first reopen Brave). But if that is true, then quitting and restarting Brave would have the same effect — which it doesn't seem to.
Honestly, memory leaks are common in software, so I really don't buy it when people say they don't need to do it. Unless it is a server where the memory footprint is arguably simpler due to not having graphics + UI + different interfaces and peripherals, I can't convince myself that a reboot won't freshen things up.