The boring answer? Warm-up clothing does exactly what it says on the label: it keeps your muscles warm and ready to perform. It’s probably more relevant for top level athletes, where match day ceremonial procedures could lead to them 'cooling down': warm-up, sit around in the changing room for 15 minutes, walk to the pitch, handshake, toss, kick off… (The same way Olympic athletes wear full tracksuits until the starting gun) (All of this has no doubt been researched by a sports scientist with shares in Nike).
It’s still relevant at the lower levels, too—like James says, as well as staying warm, there’s also the element of professionalism and looking good. But if you’re anything like me, turning up on a Sunday morning to ref the Dog and Duck after 8 pints of Guinness the night before, being warm is probably the least of your worries.
None of the warm-up kit is strictly necessary at grassroots level, but let’s be honest—it definitely saves you from showing up in your match kit, only to get soaked in the rain or being freezing cold before you’ve even stepped onto the pitch. Plus, it gives you something to change into after you've had a soaking or, you know, developed a mild case of frostbite.
I bought the zip-up top for exactly this reason. You’ll be thanking yourself when you're not standing there shivering waiting for Dave to collect £40 off the losing team.