I reset the device .Since my phone was connected to Mobile internet , so asked for samsung account login in  start screen. I rebooted device it didnt asked for samsung login again in  start screen

If internet is available then it ll ask for samsung account sign in else it won't.

Its better to sign in the Samsung for back up and more.

No even even when internet is then also some times it doesnt ask

if it detects the data connection is available, it asks for signing into samsung account

Whenever the device is not able to contact the samsung account server, this step will be skipped.  This can happen due to:
1. Mobile data/WiFi is not ON or not connected
2. Some network issue due to which device is not able to reach the samsung account server
3. samsung account server is taking a break  (down)