Adding ENERGY to the boundary layer will cause it to remain attached to the form longer. A turbulent boundary layer has more energy than a laminar boundary layer and will stay attached longer, whereas the laminar boundary layer will detach, taking the main flow with it. This is why golf balls have dimples, causing the WAKE area to be much smaller. The drag on a body is a combination of 'form' drag (having a lot to do with the size of the wake) and 'skin' drag (skin friction). Of course there is also 'induced drag' (from the wingtips) but that is beyond my feeble attempts at understanding or trying to explain.
Vortex generators will immediately trigger the boundary layer from laminar to turbulent and they will add a lot of energy to it. The flow will simply follow around the back side of the form farther before it detachs into a wake area. The smaller the wake area, the less the 'form' drag. Skin friction is increased but that is less important, in air at least.
Bottom line: an energetic boundary layer will increase the angle of attack that a wing can sustain before it 'stalls'.
This is true even at low speeds and with even with small models, regardless of wing loading or power. Whether it makes a difference on the model YOU fly depends on your specific case - the type of plane & the type of flying. In some cases, as a few folks will point out, it doesn't make any practical difference at all. In other cases it will make a lot.
Allan