All things already mentioned, but highlighting what @Hb1hf mentioned considering IoT devices.
One of the main reasons is that most IoT devices are generally not very secure, have a bad or no update cycle at all, and consequently run outdated/vulnerable software.
If u are able to put your IoT devices ('Smart' bulbs, led strips, fridge, tv, etc...) behind a/several VLAN's, u should certainly do it!
If u are unable to put them in a separate VLAN, either because of router limitations or being less tech savy, it's good practice to connect them to your guest network instead of your main network.
This will provide some sort of isolation from your main network.
Relevant read regarding IoT devices:
https://venafi.com/blog/top-10-vulnerabilities-make-iot-devices-insecure/