The foam tape is a spacer to keep the safety glass from touching the CRT directly (duplicating the spacing the removed PVA used to provide). That way if something slams in to the installed tube hard enough to break/crack the safety glass there is at least a fighting chance the face of the CRT is still undamaged.
I like to clean the glass ABSOLUTELY SPOTLESS add four small foam spacers at the edges and caulk the edge. It will stay spotless indefinitely (probably longer than the PVA lasted) if you achieved a good seal (versus potentially having to open it up every few years with tape). You can always slice through the caulk with a razor and dissolve the residue with chemicals (like 'goof off') later on if you somehow need to clean it or (in my case) discover the (spare) CRT is gassy years after cataracting it and cut thru the caulk holding the safety glass to steal the glass for another tube that needed a new safety glass...