In sports, why do athletes get signed only to get released 3 days later?

[jingramm]

Why do athletes get signed but get released a couple/few days later? Happens very frequently. Do teams readjust their rosters every week or what?
Is that like getting a job offer than you sign and they tell you a few days later that you're getting fired?

 

[midnight growler]

From the little I understand, I think its to give the rest of the team practice against different play styles.