Yeah but can't you just post an announce "i'll be in X for Y days" on SNS like the old way ? It's way nicer and a much more explicit invitation for hangout than an icon on on map ;)
I suspect that they are trying to recreate the experience of bumping into someone they know. Since the destruction of third-spaces, it is increasingly unlikely that you'll serendipitously interact with someone in an unplanned, but welcome, social environment. Leaving your location on for friends and family in this way signals something close to "If you see me, say hi". Whereas announcing "I will be at X for Y time" is a bit more heavy handed. And just knowing that isn't sufficient to actually act on the information, you still have to reach out and plan something unless you are an granular as the actual building you are in, which feels weird. It feels a little intrusive to constantly be announcing my location. Like "Hey! Hey! obk0943t! I'm gonna be in NYC just so you know!" If I just left my location on, then /if/ you care, you can find out. But if you don't, you are not interrupted with the information. Finally, posting leaves a record, whereas location sharing is always "right now". Sure, someone can use that to construct a timeline, but that takes effort on their part (and possibly malice).
Modern lives may or may not be more spontaneous (doubtful), but not everyone has the same capacity for planning as you do. Sometimes I'll randomly find myself 30 miles from home on an spontaneous side mission, and would love to drop in on friends I don't usually see, if they're around.