1. If disgruntled investors were to sue WeWork, the pitch-deck material could be referred to by WeWork's counsel in tactical maneuvering such as a motion for judgment on the pleadings, without having to jump through all the hoops that might otherwise be required;
2. Worst case, if a lawsuit ended up going all the way to a jury trial, the pitch-deck material presumably would be sent back into the jury room as a "real" exhibit, allowing the jury to review the pitch-deck material during deliberations. (The jurors might even be given individual notebooks with copies.) In contrast, if the pitch deck were left out of the S-1, the judge might or might not allow it to go back into the jury room, especially if it were a so-called demonstrative exhibit prepared for the litigation.
(I teach my contract-drafting students to draft agreements with an eye toward being readable by judges and jurors, with tables, footnotes, non-legalese language etc. — and if the contract language is understandable to a juror, then the parties' business people will be able to get to signature more quickly and are less likely to get into disputes afterwards.)