Given that there's so much debate over the probable effectiveness of economic sanctions..........?

Question

....why do you think states, individually or collectively, resort to such measures ?

Answer 1

A combination of these reasons.
1 The threat of sanctions is more effective than sanctions themselves. To keep the threat credible states must follow through when countries refuse to comply.
2 Sanctions are used to weaken a country economically, and the demands are constructed in a way that assure noncompliance.- Cuba and Iraq in the 90's
3 Sanctions are supported when military action is not feasible to avoid the charge that they are willing to tolerate evil. - Iran, North Korea
4Sanctions sometimes work - South Africa, Libya, and maybe North Korea

Answer 2

A sanction sends a message, effective or not. It shows that a nation is willing to take action, without doing anything related to war. Sanctions are particularily harmful to both sides and are decidedly not effective in all cases (take Cuba for example. Castro has yet to fall and cigar prices are not dropping). They serve as economic pressure, and as a precursor to war.