There are these moments when the sound of your amp and guitar is not enough: The sound could benefit from some extra thickening. Most people thicken up their sound during recording by double tracking: Record one part, and record same part again on another track. During mixdown both guitartracks can be used to create a fuller sound. One track may get some extra bass, or brightness from the EQ to add extra fullness and spark to the overal guitarsound.
All very well, but what can you do during rehearsals and gigs to add more body to your exhisting guitarsound you may wonder?
Why not try the same approach:?

Use a Y box (or cable): This box will spit your guitarsound into two signals, one signal is sent to your amp while the other signal is sent to the mixer. The source sent to the mixer is your direct guitarsound. This sound will be the fullest.
Next you can use a microphone to take the signal of your ampspeaker into the mixer, or you could use the line-out at the back of your amplifier.
Personally I like Line-Outs, as there are no extra microphonestands to bother with, also no extra microphone (which sometimes gets knocked over accidentily)
Line-Out signals may be a bit thinner compared to signal from microphone, but remember, your direct sound and line-out are mixed together to create one guitarsound. The overal sound should be thicker and stronger.

In case you use a lot of pedals you will need to place the Y box at the end of your pedalchain, this to make sure the sound of your pedals is also going straight into your mixer.

Experiment to see what work for you, but overal you should get a stronger and fuller guitarsound which may work better in the mix of bass and drums.

Happy experimenting.
Eddie