The expansion .. well, expands the Tech faction by adding more buildings and more needs. To satisfy the needs of their highest level residents ( the Geniuses ) you need both of the factions at the top level. It also adds quite a few less important things, gives you 3 more island upgrade slots at certain milestones, allows for energy transfer between islands with an expensive set of buildings and gives you the opportunity to buy shares in islands to take over other people's land with sheer economic power.
Overall it's neat but unless you plan to use the new stuff ( and especially the geniuses since their needs are huge chains requiring a lot of work ) it's not all that important. But it's a huge amount of late-game content.
The other DLC add new missions,versions of buildings you might have missed on the world events and decorative options.