Med- you are mostly right to say that such conventions are not binding, even if they are signed and ratified, but they are not completely useless. I will try and explain, apologies for simplifying somewhat:
The Euopean Comvention on Human Rights has been given direct effect in UK law through the Human Rights Act '98. This means that if your rights have been breached you can go to any court in the land for a remedy, whether in conjunction with a claim that another law has been breached or as a freestanding right.
Where a treaty does not have direct effect, it cannot act as a freestanding claim for a remedy, but if you have a case where a decision has been made by a public authority and you want to argue that the decision is not in accordance with the law, then you can point to the convention as a guide to how the law should be interpreted.