

My periods are super irregular, and tracking is how I try to make sense of it. My last three periods were 2 months ago, 8 months ago and 9 months ago. It’s possible that this irregularity is an indicator of poor health, so my doctor advised me to track stuff and come back if it’s more than 6 months.
Many tracking apps also include the option for tracking how heavy the flow is. The copper coil/IUD often causes heavier flow and more period pains, but for some people, this settles down after a few months. Tracking can be a useful tool in evaluating whether you’re happy with a contraception method (excessive side effects may mean needing to try a different method, like the hormonal coil).
Also, it’s not uncommon for there to be some level of bleeding at times when someone isn’t on their period. Nothing much, just light spotting. I don’t think this is super widespread, but it’s also not super rare either. It can be linked to one’s contraception. Periods are annoying enough as it is, but at least they don’t last long. Irregular spotting may happen when you thought you were safe from bleeding, but some tracking can help spot patterns.
In terms of anticipating patterns and planning around stuff, that’s definitely a thing that people do. Sometimes it’s as trivial as not wearing nice underwear when you’re due to be starting your period. Sometimes it might involve scheduling a date or holiday to avoid overlapping with the period due date. I think perhaps some people who take certain varieties of the contraceptive pill can actually delay their period (I think it’s something like taking week 4 of the medication when you’re on week 3, or something like that). I have a friend whose only contraception method with her partner are condoms (due to health issues around all the long term methods like the coil), and she uses period tracking to ease anxiety around unexpected pregnancy.
Speaking of planning around one’s cycle, I am more likely to make ill-advised horny decisions when I’m ovulating (in a 28 day cycle, ovulation typically happens around 2 weeks after the period starts). I have heard that I’m not the only one who experiences this. It’s not a huge effect, but if I’m pondering whether to get off with someone, it can be useful to know if there are background factors affecting that decision.
In short, there are loads of reasons why someone might benefit from this data. These are far from exhaustive examples. Having things on a separate calendar can be useful if apps make it easier to track things like heaviness of flow. It’s also nice to not have this stuff on your regular calendar (which may be shared with other people).
I have a friend whose cycle is consistently 28 days long, and she can predict it down to a few hour window. I think she’s a witch.