There are many reasons, here are mine:
1. Meditating with a group can elevate your spiritual experience.
2. Accumulation of Divine energy can be harnessed by all who sit together, that cannot be attained by the beginner or anyone struggling along alone by themselves.
3. There is a shared bonding experience, without having to have any interaction in an extroverted way (for those that are loners or more shy) with anyone. This can be felt as something quite profound and sacred.
4. Discipline - let's face it, its a simple practice but not easy for most. As an active society, where focus is challenged and attention is over stimulated, it can be very daunting to actually sit and begin a practice. Having to "show up" is a commitment and a discipline that has very personal rewards over time.
5. The exposure to new techniques, philosophies and styles of meditation. Diversifying your meditations at-home will keep it fresh and fun while deepening your spiritual journey.
6. Spiritual support - When you practice every experience will be different. Some days your practice may be peaceful and deep, other days you may struggle to find a comfortable position. When you are solo you become self absorbed. When you are in a group, your meditation may become focused on providing positive energy to those around you instead of solely on yourself. On the other hand, if you struggle with staying centered, the energy from others can provide you with the support and guidance you need to experience a deeper meditation.