Gravel tyre width is a moving target at the moment. At some point, we can expect sizes to stabilise but we have yet to reach that inflection point. Right now, many gravel bikes do not have clearance for tyres larger than 45 mm, but for bikes with the space, there are several newly released tyre options in the 50+ mm range.
The reason for wider tyres is simple: more volume allows for lower pressures, which combined with thin, flexible sidewalls bring increased traction and comfort as well as decreased rolling resistance. Because pressures are low, the sidewalls are less prone to cuts and punctures (if you’re skeptical about this, imagine a full balloon vs. a partially deflated one: the full one will pop if you look at it the wrong way while the deflated one will thwart even a pair of scissors).
