When you start out, you'll likely set up profiles on some of the more popular sites such as Plenty of Fish, Ok Cupid, Match.com, e Harmony, and more.
Some sites require a monthly fee, others are free for basic use, but will offer paid upgrades to allow you to use premium features.
This might clutter up your main email if you use it as your dating site email.
You might also meet some creepers online and you may not want them to have your real email so that you can avoid future harassment if things start to get strange.
Now that you've got a dating-related email account and a secondary phone number, it's time to choose what dating sites / apps you want to use.