understanding question in related IP-range when using IPsec or any kind of vpn method

[Kpax]

greetings,

Scenario:

Mobile clients (android) access the FG device from external, using IPsec / L2TP

*at the moment its works gr8, with random range that I assign such 30.30.30.1-30, point to the internal/private network 192.168.1.0/24, with pre-shared key

I would like to understand why do I need set IP-range for clients when connecting via IPsec

instead of allow the FG device to assign them internal IP via DHCP on the network 192.168.1.0/24,

I understand that except the possibility of IP conflicts is there any other reason?

thanks in advance,

Kpax