So, when you are single and moving out on your own for the first time, one-bedroom apartments may be harder to find than you think. You are more apt to find two-bedroom apartments in most neighborhoods, but as a single, young adult you will undoubtedly avoid these. You really should not avoid two-bedroom apartments because there is a lot you can do with that extra bedroom. In fact, for the extra cost in rent for a short time, you might find a really great apartment, and you can use the second bedroom for any of the following.
Some apartments do not come with storage spaces in the basement. Some apartment complexes do not come with basements! As such, renters usually have to find someplace to store their extra stuff. It is unlikely to fit all that extra stuff in an apartment unless you have a second, unused bedroom. You would have to pay extra rent on either a storage unit off-site or use an extra bedroom right there in your apartment. (Which sounds better to you?)
Hobby Room or Pet Room
Maybe you have a hobby that could use a whole new room to itself. A lot of people use the second bedroom for hobbies. Maybe you are allowed to have pets, but you want your pet(s) to stay in the spare room while you are at work. Either way, that second bedroom is worth it.
Maybe a sibling or your mom wants to come for a visit. Maybe your BFF crashes at your place after a night on the town. Whoever you know that stops by can stay in the guest bedroom for the night. You might even feel a little "noveau riche" offering people your guest bedroom when you know they are coming to town.
Sometimes, you cannot fully hack it during your first year living alone. Maybe you are too lonely. Maybe you lose your job and face homelessness if you cannot pay the rent. It happens. If you have that second bedroom, you can advertise for a roommate. You cut the rent in half, and it gives you a few extra weeks to find a job and get back on your feet (if applicable). Just be really careful and screen everyone who responds to your ad, preferably away from the apartment so that you do not accidentally give up information that thieves and intruders love.