A surfactant is a compound that lowers the surface tension between a liquid (paint) and a solid (miniature) and acts as a wetting agent.

Add some surfactant into your paint water.  It will dissolve acrylic on your brushes, stopping them clogging and keeping them clean.  

Secondly, it will act as a flow aid- making your paint slide off the brush and onto your model- great for fine detailing.

Adding some surfactant in your wet palette will help keep mould at bay- especially if paired with bottled demineralized water rather than ordinary tap water. Less stinky and again, flow aid properties added.

Other uses:

- will clean your brushes and bring life back to your dead ones.  Dilute about 50% with demineralised water and rub the brush in the palm of your hand.

- can be used to speed up layered glazes (aka juicing). The solvent evaporates quickly, leaving a streak-free glaze.  

- can be used as a dilutant for airbrush use - it has the same active ingredients as an airbrush thinner