They are used for both faster warm up and for fuel economy purposes. Couple things to note:
1. Warmer air into the intake increases fuel economy, but reduces peak power due to lower air density. To maintain stoic, warmer air (less dense) requires less fuel (Air / fuel ratio). The cooler the air, the more fuel is needed to maintain full burn and more air / fuel density = greater power output to a certain point. Back when I had my Mustang Ecoboost 2.3L (350 HP) and later Mustang GT V8 (480 HP) it was common for those cars to run 0.2 to 0.5 seconds faster on a 1/4 mile just between the hottest part of the day and the cool of the evening because of air density and humidity changes. If you noticed the snorkel for the intake of the SkyActive-G 2.5T is at the top of the engine bay where the hottest air is...because heat rises, unlike my Mustang GT where the ram air intake was lower down on the bumper and fed mildly pressurized cool air in right from outside the car. This aids in fuel economy while cruising in a low power state. Honda is going so far as to actually heat the air going into the intake with a heater element for better fuel economy...
2. Aero dynamics as mentioned by another poster. Closing off the front of the grille reduces drag as the air entering the engine compartment has to move past many very non-aerodynamic components. On a performance car, you actually vent the hood itself to get better air flow through the radiator (look at the GT350 hoods for example with the large center vent, it's purpose is to allow a direct air flow path from the front of the radiator directly past it to increase air flow for cooling). But on a daily driver, your more concerned about fuel economy than cooling the car for sustained high RPM uses.
3. Faster warm up which was also mentioned. By choking off flow, you can build up heat in the engine faster as your not transferring heat from the radiator core to the ambient air (or not as much). While the thermostat regulates coolant flow rates weather the shudders are open or closed, it cannot by nature allow circulation of the coolant AND fast heat buildup if air is flowing past the radiator. In order to fully heat all of the coolant, you need to circulate it, but not transfer heat to the ambient air. Choking off air flow to the radiator allows all of the coolant to warm up (as heat builds up in the block, pressure is built up and the thermostat opens and begins to circulate coolant).
Notice that the inter cooler does NOT have grill shudders. It's 100% open and exposed 247. We wouldn't want to choke off flow to the FMIC because extremely hot air will cause heavy knocking or detonation and consequently crack a land / groove in the piston or bend a rod. We always want the inter cooler to bring the charge temp down as close to ambient as possible. But when cruising at low loads, the turbo goes into bypass and just free spools and the engine operates as a NA (non turbo engine) where the greatest fuel economy is achieved. When in boost, your always going to consume substantially more fuel because air density is much higher and your only going to go into boost when there's a demand for a lot of torque.
When I had my 2016 Mustang Ecoboost, I had removed the active grill shudders to fit a full sized inter cooler as I was tracking the car. I can say for certain that warm up times were noticeably longer as I was able to monitor my oil temps via OBDIILinkMX and TorquePro app (later was able to do it through Ford Performance ProCal once I upgraded to their Power Pack). However I did not notice any drop in fuel economy, but the Ford Performance calibration was more aggressive with fueling (leaner) than the factory Ford calibration, so that may have off-set some of the losses.