it is different. in pure boxing, you can afford to have more weight through your leading foot, and to keep your guard closer to your face.
In all forms of kickboxing, you tend to keep the lead foot lighter, as that helps absorb impact from leg kicks- a heavily planted leg will recieve much damage to medial and lateral knee ligaments if struck on the joint.
The guard tends to be not kept as tight to the face because if a hard head kick is taken on the gloves, but the gloves are in direct contact with the head/face, the force is transmitted directly into the target- much better to have a guard that helps 'fence' and control distance, and also that is geared towards safe absorption/re-direction of head kicks.
Obviously if you are rushed, or if you are up against a fighter who favours hands over legs, then you adapt to what you are facing, but in general these are the basic differences in regards to defence and ringcraft.
The other aspect of course is having a solid base from which to throw your own kicks. You dont want to 'plant' one foot in favour of the other- it makes your kicks predictable. You have to have even weight distribution to allow fast strikes with either leg, and to maintain balance whilst doing so.