If you are using your flashes in full auto TTL, then there is no point in having a LightMeter, however, if you wish to have better control and more accurate tone, colour, brightness, contrast, shadows and highlights, then manual is the way to go and the Sekonic Lightmeter is a great tool to have.

Apologies if I happen to go over anything too basic with my answer, but the metering systems are measuring light differently.

TTL is measuring the light that is reflecting off the subject An external light meter is measuring light that is directly hitting the subject, thus the reason for the above mentioned points

In general, both sets of metering will result in different exposure settings and ultimately, different results.

A good example of seeing the difference will be to expose for a subject that has both, high levels of white and high levels of Black. Lets say, two friends, one wearing all black and one wearing all white. If you spot for white, the black will be too dark and if you expose for black, then vice versa; the white will be blown.

You can compensate for that by ensuring that you are using Evaluate metering, but at the end of the day, it is only evaluative.

With the Sekonic light meter, the white invercone when held near the subject and facing towards the direction of the camera, will break up the direct light inside this cone hitting the reflector behind the cone to provide correct exposure settings.