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External timebase correctors are created for the purpose of what I refer to as "cleaning the signal" -- meaning there are little to no visual changes that happen.
Only line TBCs embedded in certain devices (mostly high-end S-VHS VCRs) serve to "clean the image", but provide little to no cleaning of the signal itself. The TBC inside is usually integrated with a DNR (digital noise reduction) circuit or system of circuits.
This is why an optimal workflow includes both a VCR with a TBC, and a separate TBC, so that you can cover both bases.
To digitize a signal, it has to be free of signal noise, which would otherwise cause dropped frames, picture loss or be detected as a false anti-copy. The last one tends to be the errors most common, where home videos are detected as having "copy protection" from oversensitive devices. Analog copy protection is nothing more than an artificial video errors, and there are plenty of actual video errors that can trip up these detections. Analog was a pretty chaotic format. The external TBC takes care of this.
External TBCs, at most, can sometimes reduce jitter, maybe correct for a small [tiny!] amount of color noise.
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