Unfortunately, simply paying off a collection account will not improve your credit scores. The fact that you have paid the account will have no impact on your credit score whatsoever. As long as the collection account is listed on your credit reports - paid or unpaid - it will have a negative impact on your credit scores. (Collection accounts may be listed for seven years and six months from the date you first fell behind with the original creditor.)
However, there are many good reasons to pay or settle a collection account, one of which is the fact that you may be sued if you don't. That's why many people ask collection agencies to remove the collection account from their credit reports in exchange for payment. Sometimes collection agencies make this kind of offer, but usually it is the debtor who tries to negotiate a "pay for removal" deal.
Collection agencies will often respond to this request by stating that they are unable to remove the negative information. And to a large extent, that is true. The credit reporting agencies with whom they have contracts prohibit this activity. (Otherwise, collection accounts would be removed all the time and credit reports would not accurately reflect the consumer's creditworthiness.) At the same time, collection agencies cannot report information that is inaccurate or incomplete. So if you found yourself with a collection account on your credit report because you had a legitimate dispute with a creditor or service provider, it is perfectly reasonable to request that collection account be removed if you pay the bill.
While it's discouraging to know that paying collection accounts won't help your credit, keep in mind that as this information gets older, it will have less and less of an impact on your credit. That's particularly true if you are building new, positive credit references.
In addition, if any of the information reported about the collection account to the credit reporting agencies is inaccurate or incomplete, you have the right to dispute that account with the credit reporting agencies. They must verify the information with the source. If the source doesn't confirm the information within thirty days, the credit reporting agency must remove it. Some agencies will not bother to verify older paid collection accounts.
Finally, the newer FICO credit scoring model, called FICO 08, will ignore collection accounts where the original balance is less than zero. That should eliminate some "nuisance" collection accounts such as small parking tickets or unpaid library fines from hurting credit scores. Along those lines, the new VantageScore 3.0 credit score will ignore all paid collections, as well as any collections, paid or unpaid, under $250.