You don't have to "Steep" an e-Liquid if you don't want to. If you like the taste right now, go ahead and vape it.
But most e-Liquids Flavor changes over time. So if your not crazy about the Flavor right when you get it or make it, letting it sit for a few days will let the Flavor "Mature".
For people who DIY, this is Very important when making a New Recipe.
If I Whip up a Batch of something and test it, it might taste weak. So I would be Very Tempted to add More Flavoring to make it Stronger.
But since the e-Liquid is going to change, in a few days it now may be Too Strong. By letting it sit for 3 or 4 days I can make sure that the e-Liquid isn't going to change and then decide what needs to be done to it if I don't like it.
Think of it Like making Spaghetti sauce. It tastes better if you let it sit on the stove all day verses just 10 minutes in the pan.