The useful version at your size is not a hierarchy but a list of areas with a name against each. Who owns the customer relationship, who owns getting paid, who owns the work being delivered, who decides on pricing. In a business of two or three, one person owns most of them, and writing that down is still worth doing because the gaps become visible. The areas nobody owns are the ones that get dropped.
The moment this stops being optional is the first hire. A new person needs to know who to ask, whose decision is final when two answers conflict, and what falls to them rather than to you. Without that, every question routes to the owner, which recreates the bottleneck the hire was meant to relieve.
There is a second reason worth naming. Deciding what you would delegate first, before you can afford to, tells you what to document now. The areas you would hand over are exactly the ones that need written processes, and writing them early is how a hire becomes productive quickly rather than slowly.
Keep it to one page and revise it when something changes. A diagram nobody has looked at in a year describing roles that no longer exist is worse than none, because somebody will eventually act on it.
Record who covers what when somebody is unavailable, which is the part small teams never write down and immediately need. A business of three people where one is ill for a fortnight discovers quickly which responsibilities had exactly one owner and no substitute. Writing the backup name against each area takes minutes and surfaces the single points of failure before they become urgent.
Include contractors and suppliers who perform a function rather than only employees, because in a small business a substantial share of the work sits outside the payroll. The person who does your books, the developer who maintains the site, and the designer who produces your material each own something, and the picture of who does what is incomplete without them. It also surfaces the areas where a supplier has become a dependency nobody planned.
Revisit it whenever somebody joins or leaves rather than annually, since those are the moments the assumptions change and the moments the gaps cause problems. Somebody departing usually takes an undocumented responsibility with them, and the chart is what makes that visible while there is still time to hand it over rather than after the last day.