Wider bars are better at slow speeds. Look at mountain bikes as an extreme example. You have a lot of leverage. Wide bars are great if you want to turn around on a sidewalk or steer around people in a parking lot.

Narrow bars work fine at higher speeds; there's less inadvertent input due to less leverage. It's like when you ride one handed, it's most stable to hold the bar next to the stem. This effectively decreases your bar width down to about 20cm; this makes your bike more stable. Narrow bars are more stable when in a low-input, high speed situation, so something like a tucked descent at 50-60 mph, rolling along in a pack at 25-30 mph, etc.

Narrow bars also help (marginally) get through smaller gaps in races. Gives you another cm or so (per 2 cm bar) leeway when negotiating a tight situation. Also, because narrow bars have less inadvertent input, they're more stable if someone hits your arm/shoulder/etc.

I could use a 44 cm bar. I used to use one for a couple years, with a bit of time using 43s and 42s. But in general, for a while, I've been using 41s.

I used to swap bars for the winter, putting on a wider bar. This gave me a slightly wider range of hand positions, gave me another cm or two to slide my hands out on climbs and such, and it felt more "pro" (I grew up in the Lemond era, where a 44 cm bar was kind of wide, and a 46 was the schnizzle). Then, as racing season approached, I'd reinstall my regular bars. This was a pain so I stopped doing it.