If you are a winning player having as many chips as possible is an advantage to you, because when you go all-in you are capable of winning more.
Every time you go all-in with an advantage and don't have the guy covered you are losing out on $.
Let's put it this way. Say you play $50NL. You fold off your blinds (a total of $.75).
On the button you call a $1.75 raise with your 8/8 and get all-in on the A/K/8 flop vs the other guy's A/K. You're 83% to win.
If you rebought that $.75, you have 83%
equity in it so it's like getting an extra $.62.
If you hadn't topped off in two rounds you'd be missing out on $1.24 every time that situation came up.
Likewise, if you were down $5 that's $4.15 in equity (well, rake would make it $3.95 I guess).
Granted, stacking is rare, stacking off against a bigger full stack is rarer, and you're equity is often less than 83%. So maybe you're only missing out on a penny or two in EV realistically. But pennies add up.
Another thing to consider...
Lets' say you're at at $50NL table full of $40 or less stacks and you have $38 due to some bad beats.
You may think, "I only have $38 of my $50 buy-in but so what? It's only $2 less than the chip leader."
Then you win $13 by stacking a short stacker. Now you have a $51 stack and you think you're in good shape. Well, you are, but it could be better.
You see a maniac comes to the table (or maybe he was already there) and his A/J shove and outdraws the A/K caller, doubling his roll to $50. A couple hands later the luckbox maniac does it again to stack someone else. Now he has $85.
If you tangle with the maniac the biggest chunk you can take from his bankroll is your stack of $51.
Had you rebought to $50 before winning that $13 hand, you'd have a $63 stack so you could win $12 more off that maniac.
Again, these situations don't happen all the time but it's all about preparing for these possibilities. Every time they happen and you don't have as many chips as possible and you go all-in against someone who you don't have covered you are missing out on money.
So, ideally you will rebuy ASAP, replenishing right after the BB and right after the SB.
The only reason not to rebuy would be if you do not have an advantage at the poker table. If you feel spewy or out of your league, then don't reload.