Assuming that it is accepted that there is very little to no chance of being able to re-open negotiations on the Withdrawal Agreement between the EU and UK, what do Tory leadership candidates hope to gain by promising to do so?
They hope to convince Conservative MPs and members that they have a chance to convince some of the electorate who intend to vote for a Brexit party candidate to vote for a conservative candidate instead. Even if the latter (convince UK electorate) is infeasible what matters is whether conservative party members believe it to be possible.
Becoming PM appears to be an accomplishment that is prized by some politicians, no matter the circumstances or potential ignominy.
Britain's future prosperity and relations with the EU seem to be at best a secondary consideration (if at all). This seems to be discreditable but I suppose we should remember that an MP who wishes to improve the lot of their constituents is best placed to do so as the member of a party which forms a government, or failing that, as the second biggest party and therfore official opposition.
Although I personally cannot conceive how anyone can believe that it is likely that the EU will be willing to renegotiate this agreement in the next few months, maybe there are some PM candidates who genuinely believe this is possible. However, even if they believe it, the reason they need to tell us so is as above.
They hope to negotiate something that satisfies a majority of MPs, so that they can get it through Parliament - which Theresa May's agreement failed three times to achieve.
But it is a vain hope, since the EU are adamant that the Withdrawal Agreement cannot be changed.
Moreover since the ad-nauseam repeated objectives of Brexit were to "retake control of our money, our laws and our borders", it is difficult to understand how an open-border in Ireland would satisfy those requirements. However, oddly and ironically, it is Brexiters who maintain that a hard border is unnecessary.
They hope to convince their voters that the EU is blocking a better deal.
As the current deal is unacceptable - voted down three times - the only thing left that fulfills the Referendum outcome would be a no-deal Brexit.
Afterwards, they can blame the EU for any problems caused by a no-deal, if any.