The airrave uses your home's internet to create a mini cell repeater inside your home. So if you have good broadband like cable internet, you can get great cell reception inside your house using one of these.
The 2.5 is probably still fine for CDMA voice and texts, you need an Airrave 3 or 4 to get LTE data. You can get an updated model for FREE if you call sprint and qualify. To qualify, I think you just need to be on the fringe of their coverage map.
https://www.sprint.com/en/support/solutions/device/airave-support-center.html
Is there a cost associated with Sprint Airave?
Airave is free to use and there are no upfront costs. However, if you do not plug in and use your Airave or return it, you can be charged a non-return fee. Refer to next question if you wish to initiate a return or call Sprint Customer Care at 1-866-556-7310.
You may want to return the Airrave 2.5. If you ever leave sprint service, they may charge you for it if it isn't returned to them. The same as not returning a cable box to comcast.
However, I know with Verizon and Tmobile, they have WIFI calling baked into many phones these days. Sprint may be the same, you probably don't need one of these as long as you have your phone connected to wifi.