Do I need to put a dome over my seedlings?

I just started my first grow and have been a member of RIU for a little while and I have seen a lot of pictures of people putting plastic water bottles and plastic sandwich bags over their seedlings. What is the reason for this and should I do it for my new seedling?
As you can see in one of the pictures, I cut a plastic water bottle in half and placed it over my seedling. Why did I do this? I'm not really sure. I was hoping you can explain this to me.

Anyone know about domes??

The reason this is done by some (myself included) is too keep RH levels high so that the environment the seedlings sprout into is as stress free as possible , then by opening vents or making holes you can harden off the seedlings into the environment they will spend their life in.


When starting new seeds people initially tend to have low RH issues so this helps.


I only DOME my seedlings until cotilydons form, then open vents until first bladed leaves then dome comes off.



J

either way get that pot closer to your light its stretchin like son of a bytch )