Reasons to travel during off season
Not all countries in Southeast Asia share the same seasons at the same time. There are some regional differences. They are roughly divided into two weather patterns.

There are three main seasons in Thailand, Malaysia, Singapore, Indonesia, Maldives, Laos, Vietnam, Cambodia, Myanmar, and the Philippines:

Dry season: November to February
Hot season: March to May
Rainy season: June to October
There are two main seasons in Malaysia, Indonesia (Bali), and Singapore:

Dry season: April to September
Rainy season: October to March
Most travelers choose to visit mainland Southeast Asia during its dry season as it's not so hot with little rain. The weather in the hot season, consistently getting to around 38°C (100°F), and the rainy season is not so good, but you will find fewer crowds and lower prices for flights and accommodation.

Before 'online' became the norm, off season was something that a lot of people depended on to manage budget travel. Flights were cheaper, hotels were cheaper and the crowds were fewer. But with online booking platforms and offers on stays and modes of transport, people stopped looking at doing anything off season. But I believe it's time to look again at this trend and cash in on it in the best manner possible.

Cheaper accommodations
Yes, there are options of Airbnbs and hostels and other cheap accommodations but you have to agree with me that in peak or high seasons even the local homestays charge higher than usual. When I went to Ziro in September, at the time of Ziro festival of music, the most popular thing to do in that town.

Over-tourism is a genuine problem that disrupts the life of locals and it can lead to resentment towards tourists. Imagine being a local in Ladakh during the month of June, when every person you can imagine is visiting your small town, wanting to click a picture of you for Instagram likes and then just moving on without giving you a second thought? During off season it's easier to connect with locals because you are not disrupting their routines and they are also happier to share a slice of their life with you.
Shorter lines and fewer crowds
If you are visiting places like Paris or Udaipur, where you wish to go see the Eiffel Tower and click a picture of the city from the top of City Palace in Udaipur, peak season is definitely not the right time. The popular tourist attractions are full of, well, tourists and getting a picture might just be the biggest adventure you tackle. Off season guarantees lesser crowds at famous places, no irritating and unasked for photobombs and a calmer atmosphere at attractions.
Surprising experiences
Visiting destinations outside of high season means you’re more likely to see a side of that location that many travellers don’t ever get to see. Whether it’s experiencing a tropical storm in Koh Rong, Cambodia in August, or a misty morning in Halong Bay in Vietnam, you are bound to see something most tourists never will. This doesn't necessarily mean that you are missing out on things, it just means that you get to have an offbeat experience in a popular tourist spot.