To understand what are vest you need to understand what are difference between steem , steem dollars and steem power is
STEEM is the basic cryptocurrency on which steemit works on.You can transfer steem to exchanges to convert it bitcoin
STEEM DOLLARS is the currency of the Steemit. It is people get when they receive payment from curation or upvotes. It is called Steem Dollar is because 1 Steem dollar represents any amount of Steem required to reach 1 USD depending on the exchange rate at that time.
STEEM POWER is your voting power ,i.e., how much your upvote worth , higher you steem power higher your upvote.
POWERING UP means adding more steem to your steem power
POWERING DOWN means cashing your steem power to your steem, it takes 2 years to completely liquidate your steem power
to steem
VEST are share in steem power.
Vest increases by powering up and decreases by powering down
Steem Power cannot be transfered to exchanges you need to power down to convert it to steem, therefore user get reward as steem power for the commitment.
So what is the use of VESTS
As said above vest are share , just like if you invest in a company it value increases your value of share increases.
Suppose If you invest in Steem Power now with 10 steem you get 10 steem power but in a year you may get 5 steem power but if the value or market cap does not increase or go down then you might get more.
you can check your vest from
This is all I understood by looking old articles , if you find anything wrong or something i missed please mention in comment
for more details look