D.i. boxes are for impedance matching. Microphone inputs typically have a low impedance, and if you plug an instrument that has a relatively weak output into it the result can be a dull signal. It’s especially true of guitars.
So, a d.i. box has a high input impedance which gives instruments the electrical load they want to see to perform well.
Your interface has an “inst” button on each channel, which does the same job - it increases the input impedance. So a D.I. box is not necessary.
However, all D.I. boxes don’t all sound the same and some built in interface d.i. inputs sound a bit rubbish and clip very easily. So you might find the standalone you bought sounds better (though in the scheme of things it’s a very cheap d.i. )
A cheap DI box will probably do more harm than good if you’re just recording direct into your AI. But a good one can work wonders. I always use my Radial Engineering J48 when recording the bass, it doesn’t make much difference but it sounds better. The Red DI makes a big difference on a bass guitar but it’s expensive.
Thanks both. What I was hoping to achieve was getting rid of dreadful noise when recording distorted guitar. I’ve been using a limiter plug-in which has helped a lot, but it feels like a faff.
Using a good guitar cable can actually make a pretty good difference with noise. You can also make a big difference by moving around the room and finding a less noisy spot. In your case, I don’t think the DI box will make much difference, but you can always test it and see.