These days just about everything you connect to a TV has an HDMI output. Those little connections deliver pristine digital audio and video in a single cheap cable, with better fidelity than any analog jacks. If you have the choice, you should always connect your AV gear via HDMI.
But thousands of people don't have that choice. They own and watch TVs every day, some 20 years old or more, that don't have HDMI inputs. They might consider these televisions perfectly good and be loath to upgrade to a new HDTV.
The $40 Roku Express+ is made just for them. This little box, smaller than its own remote control, is packed with streaming apps including Netflix, Hulu, Amazon Video, YouTube and thousands more. Numerous other devices can do the same thing, but unless you buy an adapter, none of them work with analog video, so none can connect directly to older TVs.
The Express+ is basically an analog-capable version of the $30 Roku Express, complete with Roku's best-in-class app selection, simple interface and comprehensive search. If the TV you're going to connect has an HDMI input, there's little reason to get the Express+.
On the other hand the Express+ also has HDMI, so it might be worth grabbing if you anticipate needing the analog outputs at some point -- for example, that trip to Grandma's house. Of course, Grandma's house will need good Wi-Fi to stream video.
The only physical difference between the two tiny Rokus is the Express+ has a little minijack port on the back labeled AV out, into which you plug the included red, white and yellow cable that in turn gets plugged into the TV. It even includes a sticker you can use to affix the little box to a TV or AV cabinet, and avoid having it get dragged around by cables. A short HDMI cable comes in the box too.