If you have a niche blog where you have list of blog posts including large images, or page full of heavy contents like large size images, videos etc, then use of preloader is a good way for you to make good impression of your site users.

What Preloader does?

If you load a heavy page full of content, it usually takes time to fully load the page, depending on user’s internet connection speed, as well as server response time, or may be your site is not optimized well (if that is the case, check this article about how to optimize your WordPress blog and make it fast!). So, in that case, user will see page is loading slowly and will see half loaded images or contents which is a mess if you consider user experience. A much nice approach can be showing some kind of message to user saying that contents are loading and preparing to view, and only show full contents once fully loaded.
A preloader script or plugin is basically adds a temporary screen which may contain some animated graphics and message saying to your user that site contents are loading. And the screen disappears once contents are fully loaded, so no broken image, no broken layout for your user. Quite nice thing to have right?

Which WordPress preloader Plugin for me?

Well, There’s a lot. some free and some premium. you can simply search with keyword “preload” or “animated preloader” at WordPress plugin directory. There are some “freemium” plugins too, means it gives you some limited options for free, additional options are paid. But recently I used a pretty simple and basic preloader plugin which works fine for me. you can try it in case you want, Its called “Animated Preloader”. Its quite simple as install, activate and go type. Or, there are plenty of other options, just pick one that suits with your theme and your choice.