what are the reasons behind altcoin projects holding BACK a large quantity of their tokens with little in circulation.
short answer:
market caplets say you have 100 apples and put all in circulation. now if 1 apple is worth $1 then the true market capitalization of apples is $100 (supply * price).
now lets say you have 100 apples but only put 10 of them in circulation. now if 1 apple is worth $1 you still get the same market capitalization of $100 but it is fake because the real market cap is $10
now lets say instead of apples you have garbage that 99% of people don't want. if you inject all the 100 garbage in market the price tanks to 0.00001 but if you only let a tiny portion of it be in the market you can keep the price higher while keeping the market cap up.
now you have a shitcoin that has no value and a low price with an enormous (fake) supply and (fake) market cap that is in top 10 rank.
being in top 10 rank means newbies who don't know any better fall for your shitcoin and buy it so you earn money.
additionally you own the rest of the supply that you locked away so you can make profit from dumping small portions of it each time your shitcoin pumps.
coins like ethereum and ripple are exactly doing this.
i also understand the burning process to reduce quantity in order to increase price.
when coins burn their tokens they show that their developer had no idea about economy so he couldn't set a decent supply and had to take desperate measures to try and keep his shitcoin alive.
besides burning is not going to increase the price. it may cause some temporary hype but that's about it.