You have two options when dealing with desktop computer audio systems. All desktops come with at least integrated audio and built-in speakers. The lowest-priced desktops have the integrated audio solution. If you're interested in enhancing the sound performance of your computer, then you should look at desktops that come with a sound card separate from the basic integrated audio. Many new computers, even some low-priced desktops, are customizable and include a sound card option to boost computer audio. External speakers sometimes come with a new desktop, other times they are available as an add-on accessory.
If basic computer audio systems are fine with you then there is no reason to add a separate sound card to the new or refurbished desktop you are looking at buying. The same goes for external speakers. If you're not going to use them then don't buy them. Stick with the integrated desktop audio solution and the built-in speakers.