One of the main reasons that why you should join a gym rather than doing exercise at home is because you do not have professional advice of a gym instructor at home. Do not ignore the value of having a gym instructor who is there at every step to guide you and help you in your exercise. Of course there are other things as well which are not available at home like professional training equipment, professional atmosphere, quality food shop etc. but instructor availability at gym is the key. To support my argument, I am listing some of the important tasks performed by a gym instructor which are really beneficial for you. These are:
1- Gym instructor is there to teach and guide you about correct ways of doing various exercises. At home you may end up doing your exercises in a wrong fashion which may make your body look unbalanced and funny. But at gym, you are under the supervision of a qualified and trained instructor who keeps an eye on your moves to ensure that you do the things in correct fashion.
2- Next comes guidance of gym instructor in helping you set up a daily exercise plan so that you focus on your goals and achieve results. He understands the physical condition of your body and advises you about a good daily exercise plan which produces results for you. Such a plan you cannot design at your own and you should not because you are not an expert.
3- Next thing you need an advice on is your diet plan. Diet is again crucial for bodybuilding and a well balanced diet plan can really boost your bodybuilding. A gym instructor knows exactly that what your body needs because he is the one who has given you an exercise plan. Another important thing to highlight here is that a gym instructor can guide you about using right quality supplements. I have seen a number of guys using substandard supplements which may contain some steroids which are harmful. So you can use supplements safely.
4- Gym instructor can advise you about right rest plan and ensure that you do not over train yourself. He can understand better that which compound exercises you need to perform to make sure that you do not train your muscles more than required.
I hope you will consider benefits of having a gym instructor and prefer gym instead of trying things at home.