An SSD is one of the few things that makes a significant, tangible difference to the speed of a PC across the board compared to, e.g., overclocking a processor by 10%. You get faster boot times, faster application/game loading, and a 10-30 GB game install can only mean many GB of level data that will have to be read during the course of a game session.
You might not feel that you need one straightaway, but when you do decide to get one you'll then have to go through a full reinstall of Windows, games, save files and so on - too many people run into problems when they try and simply clone their HDD onto the SSD.
If you're building a £1200 PC now and spending £60 on the keyboard alone, it seems odd not to spend an extra £50-£70 on something which will make the whole thing much faster and save you a lot of time reinstalling later down the line.