Today WGS1984 is all your need
But
If you just use iphone browse google map,
WGS1984 is all you need.
If you are gis analyst, you are not likely use WGS1984,
You are likely use state plan coordinate system.
If you work for different city,
you will use different coordinate system.
For Los Angeles, you will use CA-zone-5
For plam springs, you will use CA-zone-6,
and so on
If you work for air force, navy,
you will use MGRS grid coordinate system
If you work for NOAA, USGS,
you will use national grid coordinate system
I can keep going thousands of coordinate system use case
why every one use its own coordinate system
instead of just WGS1984 ?
1). historically
Current existing coordinate system evolved
at least 400 years,
They are way before WGS1984.
Historic people don't have satellite
don't have GPS,
they can't get Latitude,Longitude
WGS1984 is lat,long based
without GPS,
WGS1984, does not make sense
Historically they developped different coordinate systems
to best fit their own need at the time.
Now when you process historic data,
you have to use theirs coordinate system.
Can you convert their coordinate system into WGS1984 ?
Yes, but no one want do that,
huge amount of work make no sense.
Paper map no way to change.
Keep using conventional coordinate system
is most likely you will do today.
2) technically
Military use MGRS coordinate system has a reason,
for example you are F35 pilot,
you need report your location over the radio signal.
it is not likely read wrong if you read MGRS 18S UJ 2345 6789
Otherwsie if you use WGS1984,
you will read, latitude:39.44248400, longtitude:-77.05163258
MGRS has letter and number in fixed formate, 13 digital
WGS1984, are all number,
20 digital long, one digital miss read, will get wrong.
Where to find what coordinate system they used?
For example Los Angeles
https://transparentgov.net/json2tree/esri/server/folder.html?org=https%3A%2F%2Fpublic.gis.lacounty.gov%2Fpublic%2Frest%2Fservices&url=https%3A%2F%2Fpublic.gis.lacounty.gov%2Fpublic%2Frest%2Fservices&arcgis_online_token=&timeout=9000&select_folder=18&select_layer=18&select_folder_text=Political_Boundaries+%3Csup%3EMapServer%3C%2Fsup%3E&select_layer_text=18+%26%23x21E2%3B+County+Boundaries+%3Csup%3EFeature+Layer%3Csub%3E+esriGeometryPolygon%3C%2Fsub%3E%3C%2Fsup%3E
Los Angeles use CA-zone-5
Los Angeles use CA-zone-5
https://spatialreference.org/ref/esri/102645/
La Quinta will use CA-zone-6
https://transparentgov.net/json2tree/esri/server/folder2.html?url=https%3A%2F%2Fgis.la-quinta.org%2Farcgis%2Frest%2Fservices&org=La+Quinta+CA&_center_lat=33.6634&_center_long=-116.31&_center_zoom=17&select_folder=30&select_layer=1&arcgis_online_token=&select_folder_text=school_districts+%3Csup%3EMapServer%3C%2Fsup%3E&select_layer_text=1+%26%23x21E2%3B+City+Boundary+%3Csup%3EFeature+Layer%3Csub%3E+esriGeometryPolygon%3C%2Fsub%3E%3C%2Fsup%3E
click coordinate system link
La Quinta will use CA-zone-6
https://spatialreference.org/ref/esri/102646/
