You should read this article SQL varchar data type deep dive, and specifically the Storage and performance considerations using SQL varchar section:
Because of the fixed field lengths, data is pulled straight from the column without doing any data manipulation and index lookups against varchar are slower than that of char fields. CHAR is better than VARCHAR performance wise, however, it takes unnecessary memory space when the data does not have a fixed-length. So in cases where disk size is not an issue, it is recommended to use CHAR.
Note this specifically speaks to the differences from a Microsoft SQL Server perspective, YMMV for other database systems, and not all support both
VARCHAR data types (e.g. SQLite has neither data type and only supports what it calls the
TEXT data type).
And here's some good information, in similar vein, from MySQL's perspective, courtesy of RolandoMySQLDBA - DBA.StackExchange answer to What is the performance impact of using CHAR vs VARCHAR on a fixed-size field?:
Since CHAR fields require less string manipulation because of fixed field widths, index lookups against CHAR field are on average 20% faster than that of VARCHAR fields.
Surprisingly, in the domain of PostgreSQL, it sounds like the same performance differences don't exist, and therefore the practical differences between the data types are less abundant. Please see this StackOverflow answer to Difference between text and varchar (character varying) and this DBA.StackExchange answer to Index performance for CHAR vs VARCHAR (Postgres):
...the performance of inserts and selects for all 4 data types are similar.
CHAR and VARCHAR are implemented exactly the same in Postgres (and Oracle). There is no difference in speed when using those data types.