Hypothesis: Brexit is a "meatgrinder"
The reason why people are unable to make sense of Brexit is that they keep thinking of it either in very concrete terms ("how is cross channel freight going to work exactly" etc), or in abstract slogans ("sovereignty"). Both of those are thinking far too big. The real purpose of running the Brexit process is to settle a fight within the Conservative party.
In this context, the winner is the last person to give up, and most viable tactics involve making life worse for your opponents to force them to leave the stage. An early version of this could be seen in the leadership contest, where May was the only non-withdrawn candidate. It has been ongoing in the process of putting key Brexiteers into cabinet positions: this both keeps them busy and puts public pressure on to them to be seen to achieve something. Since all their objectives are unachievable or irreconcilable, this may eventually persuade them to abandon those positions. At the very least there's the chance to physically and mentally exhaust them by making them do a lot of work.
The timewasting has already been fairly effective. All the other theoretical deals that Brexiteers claimed were easily achievable have not happened. There is now no time left to negotiate any other deal with the EU. We're down to the last three options: accept the deal, No Deal, or No Brexit.
The purpose of the stalling is to eliminate options - but slowly, so there is no time to replace them with other options which then also have to be eliminated.
The deal is obviously bad; even Nadine Dorries has realised that it means a loss of influence and the UK being forced to accept EU rules with no say. Parliament is almost certainly going to vote it down. But that was obvious two years ago, because there was never going to be any acceptable deal to all sides that wasn't making the UK worse off. All the intermediate time has been stalling.
We will eventually be left with either No Deal or No Brexit. The UK must be exposed to as much No Deal cost and disruption as possible in order to grind down the last few remaining Tory Brexiteers and their supporters in the press. The country will eventually demand No Brexit as the only remaining viable option. The Tory whips will go round in the last few days saying "Do you really want to be known as one of the people responsible for turning off the lights in the UK?"
Perhaps one of the groups worst hit by Brexit will start rioting; for example, if the Home Office starts sending out "you will be deported" letters to the EU nationals who have not yet been able to achieve leave to remain (which is most of them).
Another possibility of stalling is that May wants to hand the disaster over to a Labour or other government - but only at the very last minute. That way they get to take the blame for the consequences (of both Brexit and Not Brexit) and the Conservative party gets to enjoy the backlash.
(This is all hypothetical, but as we get closer and closer to the deadline the options really do disappear. It may still be possible that May does pick the No Deal card, in which case I only hope that not too many people die as a result of disruption to medical supplies)