According to my chemistry book, water has higher surface tension than lipids. But in my biology book it is written that "the gall forms an emulsion of water and lipids by lowering the lipid's surface tension". This will increase the difference between the two substances even more. How can this be beneficial?
There appears to be either a typo or transcription error in your textbook.
According to this Nuffield Foundation and Royal Society of Chemistry article
Molecules of most detergents and soaps are long chain hydrocarbon molecules with an ionic group at one end, usually carrying a negative charge, thus making it an anion. This charge is balanced by the opposite charge of a soluble cation, for example Na+. The long hydrocarbon chains do not interact well with water molecules, and many of them are effectively ‘squeezed out’ to the interfaces between the water and the air or the glass sides of the beaker. The effect of these molecules on the water surface is to considerably weaken the forces between water molecules there, thus lowering the surface tension.
Soaps, detergents, and bile (all examples of amphiphilic compounds) act to reduce the surface tension of water, not the lipids.
Surface tension is about how much energy you need to create new surface between not mixing materials e.g. water and lipids. Surfactants like bile makes this easier (lowers the energy required). So they can form small bubbles of lipids: micelles, which can be handled much easier by the digestive system. Another usage of surfactants to make foams, creams, etc... physical-chemistry / surface rheology is all about them.