Choke helps because only evaporated fuel burns. When the engine is cold, not enough wants to evaporate. When you restrict the air with the choke, excess liquid fuel is sucked in, so enough evaporates to start the engine. You taper off the choke as the engine warms. When the engine is hot everything evaporates so I can burn well.
In the winter they add extra light stuff, like butane, to the gas because it evaporates more quickly to help start. In the summer you don't want the light stuff because it evaporates from the tank, the hotter the day, the more is wasted. It's a waste of money and bad for the air. I think the butane from the refinery gets mixed in with LP in the summer for your BBQ, because it can't go in the gasoline and it has to go somewhere.
EDIT1: If you want to dig deeply into gasoline volatility, page 4 (adobe p12) of the file at this link really goes into it with the summer/winter volatility schedule. It's a pretty cool book. We used to send hardcopies to one of my professors at U of MN for their IC engines class. I don't know if we even have the print copies made anymore.
https://www.chevron.com/-/media/chev...ech-review.pdf