Whether or not you will need underlay will boil down to the characteristics of your carpet and your preference. Some carpets do not require underlay at all but some do. If your carpet has a felt or foam backing then generally you will not require underlay but you still might prefer to have it. underlay can change the feel of your carpet as well as prolong its life.
If the carpet you are having laid is canvassed backed, expensive or the floor you are placing it on is uneven then generally it is a wise idea to use underlay. Although it can be expensive especially when added to the price of your new carpet and fitting it will prolong your carpets’ life meaning you will get more years of use.
If you decide not to have underlay when you require it your carpets over time will look worn and tired and will eventually get ridges if placed on floorboards.