Its a question of trust

In everything that we do as people, trust is the bedrock of all human interactions. No large scale endeavour or small scale relationship can hope to succeed without trust. When a manager doubts the validity of the relationship between an employee and themselves the first steps to repairing this issue is to create an environment were trust can be earned.
Marissa Mayer has taken a lot of flack about her decision to insist that Yahoo! employees will spend more time at the office. Marissa Mayer has been brought into Yahoo! to steer the company towards sustainable profit and relevance. It is a task made all the more difficult if you doubt the loyalty of homeworkers. Gina Trapani highlighted in her pod cast (In Beta 39) that a Yahoo! source was quoted as saying "There's a ton of abuse of that at Yahoo". Business Insider.
The staff need to trust that the manger is making decisions in the best interests of all and the manager needs to trust their staff. By asking everyone to work at the office at least you can trust where everyone is. A small step towards rebuilding trust in a relationship.
The benefits for the company are control and flexibility. Staff are on hand to to be allocated tasks. Poor planning is compensated by proximity and availability of labour. For the staff, the benefits, are feeling that you are part of a team, that your joint efforts contribute to the groups well being.
It prevents the perception that those who are not in the office are gaming the system and are not accountable to the management.
Business is a question of trust, without it, no interaction can be valid.