I'm honestly do not understand if there is any difference between using a Vocal Rider plugin or a compressor for a voice track.
From what I can observe up to now, the vocal rider makes a gain adjustment causing the wave peaks are always aligned.
But it's not exactly the same as a compressor could do?
Someone could tell me if there is any significant difference between both?

I don't think a vocal rider is necessary at all. You can do the same with volume envelope automation and some compression.

Compressors, while they do "level" the signal to a certain extent, are more to
add character to the vocal sound, and to bring the vocal "forward" in the mix,
not for "leveling".

Vocal Rider basically automatically creates a volume "envelope" 
using "read ahead", based on the set parameters to keep the vocal
"even" throughout the mix.

Old school (and I still do) you would go through the vocal first and write your
own "envelope' using automation on the Volume Envelope to 
"level" the vocal based on the Waveform.  Then, depending on how it "sits" in the mix,
you'd bring in EQ's and Compressors to get it where you want it.
That also makes it so the Compressor doesn't have to work as hard.

Doesn't gain automation over volume automation provide a compressor a more consistent level to work on?

Compressors were in fact the first "levelers". The "LA" in the LA2A compressor's name is an acronym for "Leveling Amplifier".
 
Back in ancient times, before automation, leveling was a compressor's primary role. Many early compressors used in studios were actually re-purposed broadcast devices whose function was to maintain a consistent percentage of modulation while helping to prevent overmodulation, which could get you a fine from the FCC. Think of overmodulation prevention as analogous to clipping prevention in your DAW.
 
Levelers such as Vocal Rider and Wave Rider are dynamics processors, and as such are by definition close relatives of compressors, although vendors of such plugins are quick to point out that they aren't the same thing.
 
The difference is subtle, though. Compressors are non-linear and capable of fast response that can introduce harmonic distortion, which is the basis for Larry's assertion that they're useful for "color". That's true. But set a conventional compressor for very slow attack and release time and you'll get a similar results. To me, the definition of a leveler is a side-chainable compressor oriented toward very slow A/R envelopes.
 
I've read some illogical attempts by people to justify their investment in levelers. Some argue that compressors only lower levels while a leveler goes both ways, but that's not true. The reason for parallel compression and makeup gain is to raise quiet parts while lowering loud parts. I've also seen arguments that compressors reduce dynamic range while levelers don't, but that's not true either - levelers do reduce dynamic range. Some say levelers don't color sound, and that's what makes them different. However, give a compressor a slow-enough attack and it will not color sound, either. And I'm not entirely sure that levelers don't have it in them to add coloration, when presented with sudden large level increases.
 
 
 
 

Thanks Dave, learning bit by bit :)

I've frequently used vocal rider followed by compression, since they obviously do completely different things.

Kamikaze
Doesn't gain automation over volume automation provide a compressor a more consistent level to work on?

Gain is used for "Gain Staging"...setting the initial track "Gain" to hit a certain level on 
the VU Meter - typically -18 FS (DAW's by default use DBFS for the meters).  It should be
done on every track BEFORE the mixing process begins...

I never record long sections, so this isn't something I do, more read. But as gain staging is first in the chain, whereas fader riding is last and after and compressor. If you have a dynamic track, say where the it maxing at -18, but many sections of the track fall lower. Then gain automation can be used to bring up these sections, so they are more consistent to the compressor, then fader riding can be used post compressor to suit the dynamics of the track with a more consistent level to work from.

I use both compressors...usually two...AND Waves Vocal Rider on most vocals. Works great. I do the same...generally...on Bass with Bass a Rider. YMMV.

PS...another trick I use frequently is, instead of writing a volume automation envelope on a vocal track, I simply play through the track and adjust the gain of phrases, words...even syllables using the "g" command and increasing or decreasing by dbs the signal. Of course another way to accomplish this in even greater detail is to do the same in Melodyne using the gain parameter. And I do that as well.