There is an important difference that could make using
tar important under some circumstances: Besides the "metadata" that @jofel mentioned in his answer,
tar records the filename in the archive. When you extract it, you get the original filename regardless of what the archive is called.
In your case the tar archive and the file it contains have the related names
db.tar, but suppose you rename the tar file to
20-Apr-16.dump.tgz, or whatever. Untar this with
tar xvfz, and you get
db.dump. For comparison, unzip
20-Apr-16.dump.gz and you've got
20-Apr-16.dump. (Edit: as pointed out in the comments, gzip also makes a record of the filename; but it's not normally used when unzipping). A
tar archive can also contain a relative pathname that puts the extracted file in a subdirectory.
Your use case will dictate whether this kind of filename persistence is needed, or even wanted, or is actually undesirable. But certainly, regardless of compression, a
tar archive travels differently from a regular file.