Because VLAN 1 is the default used by lots of vendors, and sometimes also 2, so using 10 and 20 is easy to remember that is a VLAN and you can leave some static IP free also. Also because is not like DHCP addresses that are 1-255 but VLANs are 1-4096 so you can use some easy numbers to remember. For example I’m using VLAN 50 for IoT because the Homebridge server has 192.168.1.5 IP, so IoT is VLAN 50 with 192.168.50.0/24.
Some can argue that using VLAN 1 is also a bit less safe because it’s the default VLAN and attackers usually scan for it like 192.168.1.1 IP for modem/WAN.
Some can argue that using VLAN 1 is also a bit less safe because it’s the default VLAN and attackers usually scan for it like 192.168.1.1 IP for modem/WAN.