If you have a wedding bigger than 100 guests and lots of locations, I would recommend having a second shooter. All my collections include coverage by two photographers.
Even though I am perfectly capable of photographing the wedding by myself and shoot many weddings solo, I prefer to work with a second photographer.
A second photographer is the same level photographer as I am and they have their own business. I always work with the same team of wedding photographers who knows my requirements and match my style of photography. I am very picky about hiring people so I don't hire newbies.
If you are getting ready at two different locations, I can't be at two places at the same time. Of course, if the one partner wants to get ready two hours earlier than they planned, I would love to photograph it from the beginning till the end by myself.
During multicultural weddings, there are religious ceremonies happening at the same time and I do need a second photographer to help me to photograph it.
A second shooter also gives more variety in photography while working from different angles.
A second photographer helps with covering the cocktail hour, taking pictures of the guests while the main photographer sets up the light for the reception and takes decor photos.