Many players recommend (when building new units) to spread them out through all sectors in the planet.

My question is why?

When enemy attack your planet, he will be very happy to send even 1 stupid marine to every sector beside his main point of attack, just to have +3/+6/+9 cards for his mail battle. It is just giving your opponent cards for free.

Can someone please advise why above is still recommended?

Because you can resolve only one battle during single Mobilize order. You can't move to more than 1enemy area.

If you control many areas on the same planet, you will always have some place to resolve withdraw (for units with Cloaking). If your base is destroyed, you will have the chance to rebuild it in the same round. You also can't place new units in the enemy area (controlled by the enemy units). You won't be suprised by such Event cards like "Scouting Party" etc.

On the other hand, it's much easier to destroy even most powerfull units if they defend some areas lonely.