generally jab -> grab is a better option with higher payoff.
I'm not 100% when it knocks spacies down, but at the early knockdown percents (75+?), they'll often miss a tech and you can wavedash in and punish the missed tech with a smash, (jab reset works but they'll roll), or if they tend to roll into center when pressured, it sets up a punish
If you have the center, it's our lowest knockback option, so knocking somebody down into the corner can setup tech chase on reaction situation - pay off could be utilt onto a platform, fsmash, throw off stage.
Jab jab kick is safe on shield sometimes, mixup with double jab -> dair, double jab -> fsmash/2x fsmash, double jab -> late uair, double jab -> grab
You can use it just outside an opponents effective range, and if they try to punish it, try to punish them - maybe WD back fsmash
It flows into grab before knockdown percents, the earlier the better (within reason), I have in past opted to do jab jab kick again which isn't optimal, but is really easy to pull off. Watch schlimm shady VS jolteon for an example of the 123 into chaingrabs