Your lifestyle could determine if you must have a desk in your home or not. Are you organized? Is there a place for everything and everything in its place? Where do your monthly bills get put? A desk would be a great place to file them every month so come tax time you would have everything in one place. You could have a file for items to be paid and a file for those that are paid.
What you can or cannot do, will be determined by the style of the desk you decide on. If you purchase a computer unit it may only hold a monitor and have no room to do anything else on it. If that is the case paying bills may need to be delegated to the kitchen table every month and the bills stored in a box in the closet.
Desktop computers are in most homes these days a place to put those monitors is usually a desk. The monitor could sit on other pieces of furniture, but the height might be wrong or there might be other issues that would not come up if a desk were being used. Some of those are adjustable for comfort. You may not get the extra space for writing and such with this type unit.
The answer to your dilemma might be to purchase two units if your budget will allow and you have the space for both of them. Use one for the computer and one for your other office work. But if your space is really limited you might want to only get the unit that is just large enough for the monitor.
You may prefer to have one of the larger units to cover both areas. A corner unit or an L shaped unit would give you the space to work on the desk and have room for the monitor. You might also consider a unit with the matching credenza that would give you extra filing space. Either of these might be a good fit if you have an online business.
Spacing in your home may be what makes the decision for you. If you have an entire room to work with, one of the larger units such as the L shape or the corner unit would be great. However if all you have is a corner to use, then one of the adjustable units would hold your monitor so you could work online in comfort.
Are you on a tight budget? If so, check prices online. They can be quite competitive; savings can be phenomenal if you hit a sale or free shipping. Also if a local store has good prices online, ask if you can pick up the unit, thus saving on the shipping costs.
Take you time, as this purchase is an investment, compare prices from the different stores. Keep your eyes open for the best deal for what you want. Make an informed decision after researching it out. Then take pleasure in knowing you made a wise decision.