As a practical matter, frame data is really useful to just understand what options are best in certain situations. Like generally, if your opponent is right in front of you, you likely want to throw out some kind of a move to disrupt them. Which move? For most characters it's jab. For Rosalina with Luma out of position or dead, dtilt is faster, and hitting with that dtilt in situations my jab would have been too slow has saved games for me. You're Diddy Kong, and someone did something to your shield you think you can punish but you don't have much time. Do you try to grab? Well, grab is frame 6, and for Diddy Kong in particular, an up smash out of shield would hit on frame 5 so you can hit faster and likely do more damage by going for it (since usmash is, of course, a stronger move than grab in most situations). On the other hand, Rosalina's usmash feels pretty fast... but it's actually frame 8 so grab would actually hit faster so if you have minimal time that little bit of extra sloth might make your move unsuccessful.
In a neutral situation, you also have to keep the following general values in mind when it comes to frame data...
Moves with 10 or fewer frames of start-up are impossible to react to. If an opponent blocks these or otherwise evades them, it's only because they anticipated them before you started your attack and took pre-emptive action (you may have tells in your movement and such to make their guesses easier).
Moves with 11-20 frames of start-up are possible to react to but really hard, generally only will be dealt with on reaction by an opponent specifically looking for them. Moves on the faster end of this are physically impossible for some players to react to (human reaction time varies person to person; 11-13 frame moves are the kind of moves only some people can react to).
Moves with 21-30 frames of start-up are generally reactable but are fast enough that opponents who are surprised by them generally won't be able to react. Being surprised adds considerable thinking time to a response. Of course, you won't be able to surprise a skilled opponent too many times so make those surprises count.
Moves with 31 or more frames of start-up are generally so slow that they can only realistically hit if punishing something. Opponents will consistently react to the fact that you're doing them in almost all situations, but sometimes, your opponent has committed to some sort of movement or action that makes it very hard for them to avoid these slow moves.
For smashes as well, remember there are two reaction points: reaction to starting up the move and reaction to the charge being released. As a user of a smash attack, you can hold a charge for a bit to make the total move easier to react to but the precise timing of the move harder to react to; depending on situation, that can really catch people, and it's very important to understand your charge release frame data for this reason.
For aerials especially, remember your movement generally represents huge tells. If I'm above you and you're jumping straight up at me, I have a pretty fair guess well before you get there that you're thinking about doing an up aerial. Maybe you'll mix me up and do something else, but in that situation, the speed of your up aerial isn't super helpful because your tell of your movement is letting me understand the situation well before you get to me and start hitting buttons. On the other hand, if I do evade it, the cooldown of your up aerial will greatly dictate how good the situation is for each of us; if your up aerial has very low cooldown, you might actually still have pressure. This is why most people look at start-ups primarily for grounded moves and cooldowns (landing lag, auto-cancel windows, aerial finish time) for aerials as the most important data.