Lots of good reasons given for carrying a sidearm while hunting with a long gun, I'll add mine:
• While hunting wild pigs on a buddy's ranch in CA it's usually better to go into thick brush with a (big) handgun over a scoped rifle - it can be brought to bear much quicker and multiple shots are quicker, too. (That said, my buddy brings a 12 gauge with slugs for brush busting)
• Backup firepower - One time on a pig hunt per the above we got into a big group of pigs (the ranch has a depredation permit, there is no limit, and his dad wants to eliminate as many as possible). My friend got the first shot, the herd scattered like quail, and after I emptied my .30-30 I dropped it, drew my Blackhawk and got 3 more shots off. ( The two of us got 13 pigs from that one group
)
• I hunt deer on another friend's ranch in Eastern WA. The river bottom is choked with brush and for some reason (
) I'm often the guy picked to go thrash around in it to try and stir things up while the others are on stands on the hillsides. My '06 stays on my shoulder for that and my Blackhawk is in my hands to give myself a chance at a shot while in brush that is sometimes over my head.
• Finally, fifteen years ago I lucked out and drew a desert bighorn tag. On the 9th day of the hunt, solo, I got the ram. I came down off the mountain to my truck with my pack loaded with head and cape and my unloaded rifle over my shoulder. Unbeknownst to me, just one ridge over was a criminal subject to an enormous manhunt - the night before he had stolen a truck, killed a Nevada cop, run the truck out of gas, then hiked into the desert. One ridgeline prevented him from seeing my truck and waiting there for me, and I would have walked up to it tired, loaded down with a heavy pack, and (essentially) unarmed. I ALWAYS carry a sidearm now regardless of the hunt I'm on, whether pigs, deer, ducks, whatever. Sometimes it's OC because it might be used on the hunt, other times concealed.