The type of business someone is going into will determine if they need a partner for that business or not. There are businesses that are just good to be handled by one person alone until they have upgraded to the level where the person will need external help.
Having a business partner most times is mainly not just because of the money the person might be investing in the business, but most times the partner can also be because of the experience the person has in that field, which you necessarily need in order to take your business to the next level.
Businesses most of the time do not require business partners at the initial stage of the business, it is when the business grows in numbers that the need to employ more hands as well as business partners do set in. So, I mostly like to align the need for business partnership to the expansion of the business and not the trade itself. Whether it's a product selling business or service rendering company, the need for partnership in every big business cannot be overemphasized and it is mostly directly proportional to the growth of the business, unless of course, it the business does not Harbour the right partnership in the first instance.