This is a great way to teach students how to include important details in their sentences - beyond the simple sentence.
The first activity teaches students to include the "who, what, where, why" in their sentences. With your guidance, students label who, what, where, why in the boxes on the bases as you teach them what can be included/answered in sentences - the details. After that, students can practice writing a sentence that includes the "who, what, where and why" of the subject.
The second activity has students (or partner groups) start with a simple sentence and end with a complex or compound (more detailed) sentence. Typically I will give them a sentence such as "The girl was happy" and they write it on the line from home base to 1st base. Then they (or partner groups) add a detail each time they go to the next base and write their sentence on the line. When the students finish we have a fun time reading our sentences to each other in the front of the class.
You can teach these lessons together or in 2 separate lessons. It's a great way for students to learn how to move out of writing "simple sentences" to complex or compound sentences!