As an adhesive: MEK is best applied with a fine paintbrush.
MEK is a super low viscosity liquid that can be used to assemble plastic kits made from styrene or polystyrene, and is perfect for that job as it is quick acting and literally melts the plastic surfaces together leaving zero excess, so you'll never have glue blobs on kit joints again.
MEK can even melt the surface of a plastic rail chair so that it bonds perfectly with wooden sleepers, forming a strong bond. Additionally, offcuts of plastic kit sprues dropped into a bottle containing MEK will dissolve forming a really useful spreadable “gap filler”.

As a cleaner: MEK is a wonderful cleaner for metal parts and will effectively clean a surface ready to paint. It is also a useful additive for non-water based paint thinning (used sparingly) and can act as a very effective paint stripper for non-plastic surfaces. It is also a great glue remover, cleanly removing excess epoxy or super-glue type preparations.

Tip: Always test MEK on a small invisible area if you are unsure of the model's base material.

Cautions: Don't use plastic tools unless they are ABS or melamine which are not usually affected by MEK. Keep the lid on between usages as it evaporates quickly! Take care to keep MEK off visible plastic surfaces - if you do, just leave it to evaporate before touching to avoid fingerprints.

Always use MEK in a well-ventilated area. Do not use MEK in areas with a naked flame, fire or near flame-based heaters as it is inflammable.

Generally MEK is a naturally occurring chemical that's even approved as a food additive so general use low level exposure is not harmful to human beings, but DO treat it with respect as you would with any hobby chemicals please.