Why the database users use fixed or alterable length for their records and fileds?

Question

what are the advantages and disadvantages to each type?

Answer 1

whenever you use a field called as rollno or any such thing which is the primary key to your table and also which you know will not exceed a particular length then the database user will go for FIXED length. It will help to retrieve records faster.
eg. social security number, credit card number these will not exceed a particular limit. Morover these are of fixed length.

But when the user wants to record name,name of city, name of product etc. he/she is not sure of the length. Hence here the user will use an alterable length.

I hope this clears you

Answer 2

fixed length should mean a faster system, the computer knows what space to allocate also filtering the data can be more efficient as you know what should appear in characters 7&8 e.g. if you have a date format of --/--/--. Obviously this is then a limitiation if you are asking someone for their opinion in only 256 characters