It's an image that's been popularised in video games and film with dystopian futures and as such is often dismissed as a doomsday prepper cliche but are there practical reasons for owning gas masks?
House fires are thankfully less common than they used to be but are still a major risk and its usually the smoke that kills people not the heat.
Long before the fire has used all the oxygen in a room or people are badly burnt they are disabled by the toxic smoke house fires produce, a gas mask in this situation could help you evacuate from a burning building in time by filtering the available air.
Accidental chemical spills in industrial facilities or from transport accidents can release poisonous fumes. These noxious clouds can envelop your home or workplace if it is near to the facility or scene of the accident on major roads and railways used to bulk transport chemicals.
Often the official advice in these situations is to stay put and close windows and doors but a gas mask would allow you to safely evacuate from the affected area or ensure your survival if you do stay put.
Large scale civil disobedience protests can prompt authorities to try to disperse crowds with CS gas which irritates the eyes and lungs.
A gas mask protects you from these effects so that you can leave the area quickly and safely particularly if you have any lung related medical issues.
Military masks are often rated for CBRN (Chemical, Biological, Radiological and Nuclear) protection so are effective protection against airborne viruses and bacteria to evacuate the area so you can be decontaminated before you're infected. This is generally overkill but is another potential use albeit an extreme one.
Modern gas masks can be quite expensive, but you can buy ex military surplus ones quite reasonably and whilst the filters may have expired you can often buy new filters that will fit these masks but you may need to also buy suitable adapters such as I did here with this Yugoslavian / Serbian M1 gas mask;
So gas masks don't have to be expensive, or purely the preserve of the stars of doomsday preppers.
They can help you in several relatively common situations if you ensure they have a good seal and suitable working filters (old respirator filters will probably be effective slightly beyond their expiry date but for many common masks there are commercially available replacements).
Therefore in my opinion respirators are worth having, particularly old military ones as they also look pretty cool too if you like collecting militaria as I do...