A clean plate is vital for good bed adhesion.
Soapy water, sponged followed by a good window cleaner does the trick.
But for certain filament and plates using liquid glue or a stick is recommended.
Since this often still fails makes a mess when cleaning the plate and some people don’t like the residue in the prints we got plates that won’t need adhesives - or so they claim…
So what does this liquid glue do?
It provides the means to give troublesome filaments much better plate adhesion while also helping to get otherwise hard or impossible filament off the plate.
In most cases a clean plate is enough, so why would we bother with the glue ?
Ever lost a big print close to the finish because it left the build plate ?
I use the glue by default on the textured PEI plate.
And after well over 100 rolls the plate is still fine…
Here is how I use the liquid glue - does not mean it is perfect but works very well for me…
I have little container to hold two soft sponges.
The lid has tiny holes to let them dry while keeping the dust out.
They have different colours to make it easier to keep them separated as one is to apply the glue and one if to refurbish the plate after a few prints.
Starting from a CLEAN plate >
Apply the liquid glue so it gives and even cover, try not to dab too often as you only need to cover the plate, not soak it.
Spray some water on the applicator sponge and start wiped the plate.
You got the moisture content right when it leaves a foamy residue that disappears before you reach the other end of the plate.
Repeat this two or three times and if the sponge gets too dry spray a little more water on it.
Same story after a few prints only that you use the refurbishing sponge.
There should still be plenty of glue left on the plat unless you did a lot of large area prints.
As long as you still get the foamy streaks with the sponge all is good.
Just wipe over the plate a few times and Bob’s your aunty.
If it does not foam enough or feels very slippery use the applicator sponge to go over the plate as it is full of glue, just give it a minute after spraying the water on so the glue dissolves properly.
And ? What the benefit here?
If you ever used doubled sided toilet paper you know 
After all those rolls and mainly using a glued PEI plate I only used about 1/5 of the supplied liquid glue…
Depending on what filament you use the plate can be refurbished quite a few times.
Silk and some matte PLA filaments use up more glue, similar for some PETG blends and especially Nylon.
But even while printing matte PLA I can refurbish the plate 8 to 10 times until a cleaning is due.
Then again, I avoid touching or handling the plate without gloves…