Plungers and buckets sometimes get used everywhere – kitchen, balcony, bathrooms. That’s not very hygienic. Kitchen sinks deal with food, oil and grease; bathroom drains deal with, well, other things. Mixing tools between them can transfer germs and smells.
Keeping a clearly marked bucket and plunger only for bathroom use keeps things contained. You know exactly what they’ve touched and where they live. After using them, you can clean and store them in a discreet corner.
In a sudden toilet blockage, having them ready and not hunting for something “that might work” saves time, mess and stress.