@curtisb The full windsor is one of the most formal knots but is a very wide knot. Not suitable for every kind of shirt and tie. Skinnier ties work well with the full windsor by balancing out the wideness.
@JohnLee They can be used for all occasions! It's what suits your current collar and the thickness/material of the tie you should be worried about. Usually, the half-windsor is a good knot for pretty much all your ties.