Indeed, why would you? One could also ask, why make entries at all in each month and why not just do your accounting and bookkeeping once a year.

The answer to this question should depend on your need really. It’s about what information you’d like to have about your company’s performance regularly. Is it important to you to regularly monitor and have an overview of how your company is performing or is it okay to once a year calculate and add up all your income, your expenses and then see if you earned a profit or a loss. Even excluding some expenses or revenues from your regular accounting routines and just doing them once a year means you may end up with an unexpected result at period end. It is really up to you if you want it or not. 

One other reason to keep an accurate accounting regularly is the obligation to do so most probably arising from your local regulation, but even more so to reduce stress which could possibly arise from questions or visits by authorities. As they come, they’ll be interested in the books of your company, your accounts and having accurate accounting at all times does ensure the process will be smoother. It’s not just that, but also for tax declaration purposes. How will you know if you’ve paid (and why not even asked back from the tax authorities) correct amounts if you only do your accounting irregularly?

And last, but not least, if something is done, it’s done, whereas if something’s left for a later date, it may be forgotten. So do your entries as you come by them.