The point is that unpleasant scents can very quickly lead to negative feelings, and a desire to say no to something based on olfactory offense.
For example, if someone walks into the room and strongly exudes a bad body odor, your opinion sinks pretty quickly.
Or if you catch a whiff of food cooking and it turns your stomach, you're not likely to be interested in the finished dish.
The same goes for home buyers entering a property for the first time. The house might be clean and everything in proper order visually, but if they get hit was a nasty smell at the front door, it subtly changes that first impression. Sometimes it's not even subtle - it evokes outright dislike.
Because every sense can impact a buyer's impression of your home, don't neglect to eliminate day-to-day smells that may turn buyers off - pets, cooking, hobbies and other things you might not think about. Wash or change your pet's beds and put toys away. Try to avoid cooking foods with strong scents, particularly before showings. And consider the impact a strong chemical scent might have on someone walking into the home before you start staining your latest wood project in the basement.
Besides emotional response to smells, some people also have scent sensitivity. There's debate about whether this is becoming a more common problem, an imagined problem, or if awareness and acceptance is just growing in recent years. Regardless, I have personally had clients who developed migraines if confronted with strong scents, so it's also good to avoid air fresheners, scented candles, and especially something like incense.
The best bet is a nice clean and odorless environment, as much as possible. If you're not sure, get someone from outside the home to take a walk through and let you know if they smell anything.
One word of caution here: if there are permanent or more serious smells, make sure you're dealing with the source. For example, simply covering up a musty smell with air freshener without finding its source could lead to lawsuits later if the buyers discover a leaky roof and mould in the attic. Reduce and eliminate day-to-day odors, but if there may be a more serious problem behind it, try to find it and fix it. Or at least disclose it to reduce exposure to lawsuits later on.