To prevent stalemates. (im not partial to it, as I enjoy drawn out battles)
A stalemate is when neither side can win. When an attacking team cannot establish in the first objective, that is far from a stalemate; that's getting dominated by the defenders and the loss will be decisive. So it's not really the stalemate that it was intended to prevent.
On an attack/defend map, it was/is possible for attackers to roll through quickly and get a speedy victory, but it wasn't possible for defenders to get a quick victory. So one of the things that lockdown introduces is a mechanic to equally reward solid defense with a quick win the same way that weak defense can be punished with a quick loss. In the former game it was not possible for the defenders to achieve a decisive quick victory because the only way to earn a defensive win was to hold throughout the round.
I think of the reinforcement of the concept of pushing at objectives as a secondary intent of lockdown. All but the most obtuse of players will learn after a few lockdown losses that the game is about objectives, and it is more fun when you make an effort to capture them.
I think of preventing long pointless battles is at best a tertiary intent of lockdown. I'd been in a lot of good long attack/defend rounds in RO:Ost, but I'd also been in a lot of bad ones where it becomes clear that roughly the second half of the round timer is a pointless exercise in futility. Sometimes on attackers those kinds of rounds would've felt better to cut our losses and just move on to a new round or a different map, like a retreat/rout as someone said.
But, contrary to what many imply about it, it doesn't wholly eliminate long rounds, as usually it is only the first one or two objectives that have the lockdown timer associated. Once the attackers defeat the lockdown, it is very possible to have a long battle for the remaining objectives, and that happens pretty often.
I think lockdown has debatable aspects that may need tweaking, but I'm not decided on any of them. Take reinforcements for example. Does it factor reinforcements into the lockdown? Say the attackers are not getting the objective but are taking huge advantages in reinforcements to where if they were given enough time to get the objective they will probably win because the defense can't outlast them reinforcement-wise as they attack the remaining objectives.
Also many people mentioned overtime or something to allow for the
effort to get the objective. It always stinks as an attacker when your team finally gets it together enough to start having a real chance at capping that objective but then the buzzer goes off and you lose just before it switches over.
Also maybe lockdown just needs to be on a somewhat longer timer.
But, I haven't decided on these aspects because I like the basic idea of a quick decisive victory also being available for the defenders, and most of these latter points significantly undermine that principle by just extending the amount of time it takes to get such a victory. After all, a capture is a capture, and anything less than that is a failure to capture. If lockdown is overbalanced in favor of the defenders and gives them a clear demonstrable advantage reflected in win/loss statistics, then yea it probably needs slight tweaking, but I'm sure the devs have been looking at this (I think someone mentioned they would try to address lockdown).