Withdraw and Refit refunds 75% of the resources (manpower and fuel, I believe). It's a pretty nifty ability since you can exchange your early vehicles for something better as the game progresses and they get obsolete.
I've been playing around with this commander and found the WC51 to be extremely potent early on vs OKW players who don't make early Raketenwerfer. If you put a rifleman inside a WC51, you can pretty much drive up and take out sturmpios / volks (unupgraded) very quickly without taking much damage. The jeep can take 1 hit from the Raketenwerfer and survive, but it's a bit sluggish / slow to move around.
The M3 is awesome. It can take 2 squads so you can customize it to fight whatever is on the field. If you arm both riflemen with 2 bazookas each, it can be an uber, makeshift Jackson doing 240 damage each salvo. Arm them with BARS and you get a makeshift KV8 that'll mow down infantry very quickly. It can't take much damage though (320HP), but it's fast enough that you can circle tanks around.. I've taken down an unsupported Tiger to a tiny bit of sliver health left this way (happened to catch it on its own as it entered the field and his support wasn't nearby). Plus you have the ability to reinforce.
If you're quick enough with your micro, you can also "get out" of sticky situations to save your vehicles. Let's say you're about 1 hit away from getting your M3 destroyed, you can simply hit T to abandon the vehicle and enemy AT will stop targeting it (since it's abandoned) and fire at your infantry instead. Depending on how quick their micro is, they can still attack ground to take it out, but that still gives you a second chance to turn the tide.
One thing to note.. the shortcut for the M3 overdrive ability is "D", which is the same key to unload your troops. So when you press that, your M3 will get into overdrive but it'll unload everyone. Manually click on the button instead.