If you have a progression in one key (let’s say I-iii-V-ii in C Major) and you want to go to the same progression in different key (let’s say i-III-v-ii in E Minor), Scaler will sugest a possible transitions/bridges that you can switch with arrows.
Similar as progression above but it assumes that you do not want to keep the progression. Simply “change the key and do whatever” 
I do not know how to use it “properly”. If there is even such thing as “properly” in music 