Robotics Factories reduce the time taken to construct buildings (but not defensive structures or research times). Robotics Factories have less of an effect for higher level upgrades. For example, the first level of a Robotics Factory will cut building construction time in half, but the ninth level will only cut the time by 1/10 of what it was at the eighth level. If a building takes 1 hour to build before you build your robotics factory, it will take 30 minutes after the factory is built, but if a building takes 1 hour to build with a level 8 robotics factory, it will take 54 minutes to build after the level 9 factory upgrade is built. Therefore, it is rarely worthwhile to upgrade the Robotics Factory beyond Level 10.
When your factory is upgraded to level 10, you will be able to build a Nanite Factory (assuming that you have researched Computer Technology level 10).
Many players prefer Nanite Factories because every level halves production time for buildings as well as for fleet and defense. However, Nanite Factories have a large base cost, so you will not be able to afford one until later in the game.
Hint: Some players choose to free up fields by partially or totally deconstructing robotics factories after building nanite factories to their desired level.