The 5 lb Power Tank is great for overland vehicles running up to 33 in tires. It is lighter and more compact than the bigger size Power Tanks while still giving you 12+ tire fills depending on how much air you use. For longer overland trips, this size works great in conjunction with on board air. The Power Tank is kept as a backup in case you need to reseat a tire, run air tools, or just need to air up as fast as possible to get out of the rain. The fact that it's portable means that you can help other folks without having to reposition your whole rig.
This is also a great size for Side by Sides running bigger tires or going on longer adventures. The last thing you want is your day to end because you got a flat or your tire slipped off of the bead.