An ISA is something that everybody who has any savings should have. There are a number of reasons for this but by far the most important is that as a part of a government scheme to promote saving money, an ISA has an allowance, an amount you can put into every year, and that amount will accrue interest which goes completely untaxed. Although this makes it more limited than other investment opportunities, it is still a very profitable option and worthwhile for everybody to participate in.
ISAs are also a great introduction to making and managing your investments in savings for those who currently have little to no understanding of how to go about it. For instance you can choose whether or not you want your funds to be used for cash or investing, or a mix of the two options.