Specifically to help pop income and / or food producing empires:

The types of orbital infra would include:

1.'habitats' + a certain amount of pop
2.'agrispheres' + a certain amount of food production
3.'weapons platforms' + a certain amount of orbital lasers

Unlike ground infra (aside from lasers), orbital infra could be destroyed (to some extent) by incoming bombers.

Weapons platforms would defend against incoming bombers, figs, and transports. The weapons platforms wouldn't destroy all incoming bombers, figs and transports, however, only to a maximum percent.  Ground based lasers would still be necessary to defend the planet.

Example:
Say a fleet of 1000 bombers, 10,000 figs, and 1000 transports were sent against a planet which had just the right amount of weapons platforms for defense.   The initial bombing phase of the attack would leave 1 weapon platform remaining (no other orbital infra would have been destroyed because there had been enough weapons platforms to defend them).  Because the bombers had to go after the weapons platforms first, none of the lasers on the planet were destroyed.

As such, the last remaining weapons platform would counter-attack and destroy 100 bombers while the lasers on the planet would destroy 70% the remaining bombers (700).   I think maximum destruction of 70% is still the current rule? 

Anyway, the single remaining weapons platform would also destroy 1000 figs of the 10,000 incoming figs, and 100 of the 1000 transports.  The remaining 900 transports would sneak through, only to get destroyed by the ground-based lasers that, again, wouldn't have gotten hit at all during the bombing phase.

As there would be no build-space limit and thus no OB cost associated with orbital infrastructure, it would be economically feasible to rebuild the 9 weapons platforms that had been destroyed.  The alternative of rebuilding lasers on a 2000% OB planet would be just as expensive.

Again, in spite of not having any build-space limit or OB cost associated with orbital infra, they would, however, be very expensive - not really economically feasible until the planet is already 1000% or 2000% over-built.

Basically, the idea is that cost of building orbital infra would become tenable only when the OB cost of building more LQs on a planet has reached a certain point (this point would have to be determine by complex maths that I don't want to do).

What would the benefit be of orbital infra?  Specifically, they would allow for:

1.  Empires producing a tremendous amount of food on only a few planets.
2.  Empires producing a tremendous amount of pop income on only a few planets.
3.  Empires producing both a tremendous amount of food and pop income on only a few planets.

The benefit of doing so on fewer planets would be tremendous, as it would lower their NW relative to empires with many planets and little to no orbital infra.

Overall, orbital infra would help small fams continue to grow and remain a force to be reckoned with in spite of their lower planet-count.  This would be especially beneficial for fams who have been blocked from expanding their territory by larger fams.