There's always more to be said and other angles to write from, but I actually wrote an answer to Is it true that her husband was executed first and then they kept her around because she was related to Austrian royalty? some time ago. (Quoted below.)

I'd suggest a subtle shift from "they wanted to get rid of her but had a reason to keep her around" to "they didn't know if they should get rid of her or keep her around".

After Louis was separated from his wife, sister, and children, the rest of the family was still kept together and in relatively luxurious circumstances in the Grand Tower, with fresh food, nice furniture, silver dinnerware, and flush toilets. Louis himself was in a decent position until the discovery of the "iron chest" where he had kept his correspondence, which wasn't at all incriminating, but was enough to make him look like he was keeping secrets, and which kicked off the true preparations for his trial. Said trial was obviously going to end with him found guilty, but it was not clear whether or not he would be executed (though he made up a will while preparing his defense, since one revolutionary faction certainly wanted him dead rather than exiled to America): when they voted on that measure, it only passed by a narrow margin.

Louis' execution in January of 1793 sent Marie Antoinette into deep mourning, but it resulted in security letting up to some extent. Louis had been the official ruler of the country, and while it was widely held that his wife had given him misleading/bad/self-serving counsel (the traditional reasoning for unpopular measures, though it was usually applied to royal mistresses), she wasn't seen as having done anything illegal at the time - queen-consortship was essentially a powerless position. Rather than being held as a hostage, many assumed she and her children would be traded to Austria in return for French captives; there was precedent for former queens being taken back in by their natal families, and none for the execution of a consort in similar circumstances. At the same time, some radicals had called for her violent death prior to Louis' trial, and her nephew the Holy Roman Emperor seemed uninterested in putting in the time and diplomacy toward getting her out. Eventually some effort was made to do this, but between the Emperor not wanting to make any concessions in return and Marie not wanting to leave her son, it failed. (Marie had no real political standing as queen consort, and due to the succession laws in France her daughter, Marie Thérèse, was unable to transmit a right to the throne to potential children, but little Louis Charles could of course be seized as a future king by the allied forces or royalists.)

By March, the war was very much not going in the French forces' favor, and Robespierre was bringing up Marie Antoinette's "crimes" and need for a trial, perhaps as a way of rallying revolutionary morale. When the more temperate Girondin faction were arrested and scattered, the rhetoric became closer and closer to reality - security was increased and Louis was separated from the rest of the family. A month after that, an Austrian victory caused the government to again use her character as a scapegoat and her fate as a motivator. By this point, there was no question of her having a value other than as a reason for a triumphal prosecution and execution.

So the tl;dr is that they did not make a conscious decision to keep her around after Louis XVI was executed to serve a purpose, they had her because she was already imprisoned and they needed to do something with her. It appeared that she would be most useful as a sacrifice, someone who could be charged with some really heinous crimes that would prove how deceitful and debauched the royalty/aristocracy were and then justly beheaded - so that's what they did.