Their primary aim remains to gain power in a parliamentary system and hold and exercise it accordingly in order to further particular political aims and goals.
The second part isn't optional.
Corbyn is frequently portrayed as representing ideological purity at the expense of electability. I'm not sure how true that is, but it's certainly the case that he - and his support - are more concerned with pushing ideas than winning power. Ideological purity is not a viable position for a functional political party.
Blairism represented the other end of the spectrum, the extreme pursuit of electability over ideology*, but this is just a fruitless and ultimately self-defeating. Parties and politicians that don't seem to stand for anything lose their base, disillusion their members, and eventually lose the support of a public who, rightly, just see them as blown in the wind without standing for anything. So electability at all cost is also not a viable position for a functioning party.
Major parties need to win elections to achieve their aims but they also need to stand for something.
* - Although Blair said that he wouldn't support Corbyn's ideas even if they were electable so I'm not sure how true that is of Blair himself.
Hence the latter part of that sentence. But instead of limiting it to the rather vague aims and goals, which is all Corbyn's good for (well, that and criticising the blooming obvious), it also includes the emphasis on the exercise of responsible power held on behalf of all voters, not just party members. That's what you stand for first and foremost.
Ideology of a political party in the UK must be a flexible framework robust enough to get elected, bind the party as a working unit and shape policy through expert advice and empirical evidence. It cannot be left as an unaccountable ethereal mess until it's time to pin down a manifesto, worse still -- a working government. Therefore an ability to change one's approach based on data and compromise, when necessary or forced by conflicting priorities and limited resources, is vital for anything to get done, and to stay so. People management goes without saying -- Jezza's awful at it, Boris Johnson level of awful.
Tony was terrible at the evidence part. Corbyn's letting the side down on everything else. Brown was too much of a technocrat. But it's a matter for the party to select, train and manage its leadership, not the nation's. Picking someone competent and electable is not an impossible task. Picking someone comforting is a recipe for disaster, since at that stage you're in identity politics without a paddle or compass, casting about for events to confirm your bias and blame someone else for your troubles.
Having said that, brand is important. One or two big policies helps, but I challenge you to find many a Labour voter who can go into detail on their party's manifesto and inform you of what the current goals and aims are (ditto for swing voters), and vote accordingly. (Likewise for other big parties.) The Tories manage branding for good or ill rather well even when the mask slips; what's Labour's problem? Not that they are short on Oxbridge-educated PR people with connections in the media. Incidentally, tis also why not treating the majority of your MPs like dirt also helps -- for lack of anything else, they're the brand in a representative system.
Indeed -- what's Labour's brand? What do they stand for now? What the public sees is a troubled leader fighting his MPs over philosophical points he can't communicate to the outside, and MPs not able to perform their duties, fighting internecine battles with other stakeholders of primary interest only to a few active members. The brand is so weak and absent from the public sphere, Theresa May is lecturing them on social justice and responsible capitalism -- it has to be seen to be believed!
Again, parties and MPs stand for their constituents as much if not more so than their beliefs, caveat being their judgement and not the will of the mob is what's employed as democracy. (You can be more ideological in a proportional system, I admit. But that's not what we have.) There are no great ideals without the graft. And Corbyn's dilettante martyr act can only go so far. In fact, it's peaked months ago. Whether it'll be possible to rebuild the party afterwards as an effective force is anyone's guess at this point.
Corbyn clearly isn't a competent leader but it doesn't sound like Owen Smith would be any better. 
