Why Collect Japanese Cards?
Aesthetics
Japanese cards just look better.
Once you've got your hands on Japanese Pokemon cards, you'll never look at English cards the same way. Unlike the yellow border on English cards, Japanese cards have silver borders on them. The subtle and neutral colour acts as a gentle backdrop, putting more emphasis on the artwork.
Quality
Japanese cards have better print quality over English cards.
With rigorous quality control, Japanese cards tend to have fewer print issues such as off-centring, cropped cards, surface/edge damage etc. English cards are more likely to have factory print issues, especially more apparent in more modern sets like Chilling Reign. Yes, there might be a few printlines or slight off-centred cards here and there; but the overall quality of Japanese cards is still far superior than English ones.
Rarity
Japanese exclusive cards.
A lot of Japanese promo cards don't get released in English. With dozens of collaborations and Pokemon Centre openings every year, promo cards such as Shibuya Pikachu, Mario Pikachu and the Scream promo set are only printed in Japanese. You would not want to miss out on these amazing and iconic cards.
Price
Japanese booster boxes are cheaper than English boxes.
A Brilliant Star booster box is about 180-200AUD, while its Japanese equivalent: Star Birth, is selling for 100AUD. Despite the price difference, you're still getting (roughly) the same amount of hits. A handful of holo rare cards, 3-5 GX/V cards, and 1 guarantee secret rare card.
SMALL SET
Japanese sets are smaller and easier to complete.
Each Japanese Pokemon set typically has fewer than 120 cards (with the exception of holiday sets such as VMAX Climax). Smaller sets mean it's easier and quicker to collect and complete sets. If you feel like mastering English sets is too tiring and tedious, try completing a Japanese set. It'll feel more achievable and rewarding.
EARLY RELEASE
Japanese cards are released earlier than English cards.
Japanese sets are usually released a month earlier than their English counterparts. Buying Japanese cards means you get your hands on newer cards as soon as possible. If you're collecting English cards as well, the earlier release of Japanese sets gives you an idea of what to expect in the upcoming English set.