Because whisky has a relatively high alcohol content - it must be an absolute minimum of 40% alcohol by volume (ABV) to qualify as whisky - it doesn't react with air the way that weaker drinks, such as wine for example, do.
Whether you decide to decant your whisky or just store it in the bottle that it comes in, the flavour won't change any.
Decanting whisky is a purely aesthetic decision - and there's no doubt that a nice whisky decanter looks great, especially when it has a nice single malt inside it!
If you're looking for a gift for a whisky lover, a whisky decanter is a great choice. You can get a decanter on its own or choose a decanter and glass set. Either option will be gratefully received by any whisky lover.
You can find a huge variety of different styles - and at a wide range of price points. You can get very acceptable decanters for less than $20, or you can pay several hundred dollars if you wish.
Personally speaking, I prefer decanters which have a fairly simple styling without to much engraving. That's because I like to be able to appreciate the depth of the whisky's colour. However, others will have different tastes and that's no problem, you should find something suitable for the whisky aficionado in your life quite easily.