Technical pre-sales is a crucial function in selling many business-to-business solutions. The pre-sales function is often selling trust to the client; the client needs to know that "the thing" will happen if they buy the solution. Most importantly, they need confidence it will happen on-time and on-budget.
This is a huge part of the cycle - many sales people simply do not have the depth of knowledge that is required to talk in great detail about the minutiae of a technical solution but this knowledge is essential to a successful project.
As with nearly all meetings, the client will likely search for the individuals that are coming to a pre-sales meeting; if they find the profile of an individual who is "all over" a subject, that will markedly improve their sense that all will be well if they choose that vendor.