reason for the alledged assassin, Oswald to use his own building, the TSBD
1. Oswald was apparently confident that his rifle is untraceable to himself and he can therefore leave it in the building aftter shooting.
2. Oswald has decided that the best spot to shot at the moving JFK limo is from a position high above and behind the limo.
3. Oswald is familiar with all the floors of TSBD, has access to all the floors as part of his routine job, knows the staircase layout, the elevator locations, and most importanly, knows the work habits of the workers and where they likely willl be at lunchour between 12:00-12:30, thus the 6th floor becomes a particulary suitable location.
4. Easier for Oswald to get the rifle into his own building in a package and up to a higher floor he is familar with that he knows he can assemble rifle without likely to be interrupted, such as the 7th floor. And able to do this with least probability of arousing any suspicion of any other persons in the building, while moving up and down staircases.
5. Oswald may have thought he could get out of TSBD without being seen police, but that plan was interruupted by DPD Bakers early entry into TSBD.
6. It is unclear if Oswald had planned a plausible alibi for himself and if that involved being seen in the 2nd floor lunchroom at 12:13 but also attempting to display himself on the 6th floor with a rifle, just 2 minutes later. at 12:15 which would be a ratther risky thing to do, since someone might photograph him in the SW window holding the rifle at the time Arnorld Rowland would have seen him there.