Yahoo's employees come back in office

Yahoo chief bans working from home. It is been announced by Marissa Mayer (Yahoo's chief) last Friday. She argues to be a contender in the web space, have to work into real life. For this reason, it is necessary that company's staff have a their "seat" in the office. Nevertheless, I have a matter:  this decision is relevant in reference to social network (e.g. chat, to work side by side, meeting new people, etc) but how will she write off fixed costs (e.g. new offices, new office furniture, more electricity etc)? and then the employees are satisfied to work at home because there aren't there some usual "office distractions" as coffee, chat with colleague, loudly talk etc, so why did Marissa Mayer make this decision ? Which will be effects of that?