If mac has the built-in backup tool time machine, why we still need mac data recovery?
Time Machine is a bit different from third party data recovery software. Here's how they work:

When your computer is connected to an external hard drive and Time Machine is turned on, every hour it backs up the files on your computer (or any files changed since the previous backup). In this way, you always have a copy of everything, including versions going back in the past (depending on how large the external hard drive is). Then if you lose a file, you can grab a copy from this backup. (As a side note, the latest version of OS X includes a Versions feature that makes locally stored backups of older versions of files, so if you change a document, you can still see what it used to be.)

Third party data recovery software, on the other hand, is what you use when you do not have an actual backup to rely on. It's also not very reliable. What this does is it tries to scan your Mac's hard drive's empty space for files that were deleted. This sometimes works because even though files are deleted from the hard drive, the hard drive isn't necessarily overwritten, so some deleted files may still be retrievable.