Have you ever been out for a jog, preparing to cross a street on the walk symbol, only to have a car pull forward and block the crosswalk?
It's dangerous and common. If you're already in the crosswalk, it is illegal. It puts you at risk.
The trouble is that drivers often do not think about pedestrians. They may see the crosswalk, but they really just think of it as another part of the road that they're allowed to use. It doesn't dawn on them that you may need to cross between those lines until they actually see you step off the curb. By then, with a red light ahead and another car behind, there's nowhere for them to go.
Another issue is that drivers need to see the cross traffic. They may stop behind the crosswalk and then slowly move forward so that they can see if anyone's coming.
For you, this raises all sorts of safety issues. A driver entering a crosswalk that you're already using could hit you. A driver sitting and blocking a crosswalk that you intend to use could force you to either wait in the traffic lane or move out of the crosswalk and weave between stopped cars. The latter can hinder your visibility and the ability of other drivers to see you, putting you at tremendous risk as you cross at an unexpected place.
If you walk or run often, you've probably seen this play out numerous times. If you get injured, be sure you know what legal rights you have to compensation.