If you are having a small guest list, less than 75 people, you can probably get away with people sitting wherever they want. Also, if you are not having a formal meal but a more casual cocktail style party you don’t need assigned seats.
When your guest list is rather long (75+) it can get awkward for guests to have to find their own seats. Some may find it hard to find enough seats for their “group” at one table. And when I write “group” I don’t just mean a party of 2 or family, but rather the group of guests who would like to sit together -- your college friends, your mom’s siblings and their spouses, the groom’s colleagues, etc…
While creating your seating chart can get tricky, and stressful, it’s a courtesy to your guests to create one, even if you know that one or two people may not like their seat. Just try your best to make it work for as many people as possible.