There are different kinds of partitioning tools. Some partitioning tools (e.g. *nix fdisk, *BSD disklabel) only change where partitions start and end. If there are existing filesystems on the drive whose starts and ends overlap the new partition sizes, tough, you wreck the existing filesystems and lose the data. If you use these tools, then backups are essential if you want to have a copy of the data on disk.
Some newer and fancier partitioning tools, e.g. Mac OS's Disk Utility, are closely coupled with the local operating system. They have enough knowledge of the filesystems on disk that they can edit the filesystem, altering superblocks and summary information and whatever else, and moving data around to match the new partition sizes. Tools like this let you repartition a drive without losing data, mostly. You could still have a system crash or a power outage in the middle of the process and end up with a jumbled drive. Also, your partitioning utility could have a bug in it. You are gambling everything on the drive that the repartitioning process goes well. So even with the fancy tools, it is wise to have backed up your data.