The manual writes:
In contrast to CHAR, VARCHAR values are stored as a 1-byte or 2-byte length prefix plus data. The length prefix indicates the number of bytes in the value. A column uses one length byte if values require no more than 255 bytes, two length bytes if values may require more than 255 bytes.
That is, the only effect of specifying a shorter length limit is that it can enable the database to use 1 rather than 2 bytes to store the (byte) length of the text.
Specifically, this means that a
VARCHAR(1) and a
VARCHAR(63) are always stored and retrieved in the exact same way (assuming an database character encoding with at most 4 bytes per character such as
utf8mb4). Depending on the character encoding used, a VARCHAR with a higher limit may require 1 additional byte to store the length.
VARCHAR length limits have negligible performance impact, and it many cases have no performance impact at all.
The database provides
VARCHAR length limit support not because the database needs a limit, but because an application might. For instance, if your user interface has room only for 20 characters, you may want to express this constraint in the database depending on your application architecture.
Historically, this was an important feature because many popular application programming languages such a COBOL used fixed length strings, and databases were often shared by many applications. Nowadays, applications usually handle overly long strings gracefully enough that such constraints are no longer needed (or at least, no longer needed at the database level).