There’s a few things at play with noise, and unfortunately there’s no easy answer to your issue.
Easiest and best bang-for-buck: seal your windows up tight. If there’s little gaps for vents, the window doesn’t close tight, or things like that, they will leak noise.
Most effective: replace windows with solid frames or insulation packed extrusions, double glazing using laminated glass, and make the inside pane a soundstop type of glass. Make sure any opening windows have seals.
The curtains probably help with some very high pitched sounds, and there may also be a placebo effect at play, but they’re going to do bugger all I’m afraid. Noise is just vibrating mass, and it’s harder to transmit vibrations through dense mass. So yes, a curtain is better than nothing, but relative to your glass and walls it’s closer to air.
If noise is an issue for sleeping, the very best thing to do before shelling out loads of money on snake oil curtains is to try a few different ear plugs.