It might be considered old-fashioned, but many hosts and hostesses still prefer to seat an even number of guests for a special dinner. If one person bows out, they may resort to calling a friend or relative to take the empty seat. One more for dinner can be a distressing chore, and they will do what is necessary to get a person to accept. They might use guilt, or they could work on being persuasive to fill that unwanted and empty seat.
The person on the receiving end of this late invitation is often not flattered, but they generally do have some type of relationship with the person calling. Even the most modern people hosting a dinner will want a mix of men and women, and it could be more like a date than a person wants. Knowing they will be expected to make conversation with someone they have never met can be a barrier, and they will need to overcome it to fulfill their duty as a guest.
While they could be a bit upset or even feel they are being used, the person they will be seated with might not feel any better about the situation. They will often do their best to make the time go smoothly, and the last minute guest should at least appreciate their effort. For those with an open mind, it could turn out to be a fun way to pass an evening. There will be conversation shared with many of the other guests, and they could find it is much better than sitting alone at home with a take away meal.
Meeting someone new on in this type of situation will not always lead to another date or relationship, but it could be a starting point for two single people. They could find they have more in common than just being seated together, and their conversation could take them places they never expected they would be going.