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.

Same reason as assigning larger networks than you need or leaving free spaces between them. You may want to put some things close to each other because they logically go together. But some things that go together don't exist yet, so let's reserve the space.

(Can't speak for everyone of course, but that's why I'd use 10.0.10.0/24, then 10.0.20.0/24, etc. Now "same kind of thing next to it" can have 10.0.11.0/24)

Originally, so you could group related VLANs together. e.g. VLAN30 is Marketing, then later you need a second marketing team so they have VLAN31. If you’d had VLAN1, 2, 3, etc, you couldn’t do this.

That everyone does it - even on small home networks - is just convention.

On some devices (e.g. CISCO), ID 1 is reserved, so starting at powers of 10 keeps it nice and even and allows for insertions (same logic as line-numbering in BASIC.) I assume 10 seems better than 100 (or even 1000); those just seem crazy high.

At least in our case, this allows this space:

172.16.0.1 to 172.16.9.255

To be available for non-VLAN DHCP, static leases, and internal devices. Not sure if that's why others do it this way, but it made sense for us.