Yeah if you're still new to the game it might be unclear why. To summarise:
1) He's one of the only champs in the game that has an AOE attack on all three of his abilities. I'm sure there are others, but the only other one I can think of is Bellower
2) He has a *ridiculously* high base attack. I don't know if it's the highest, but it must be up there. That makes the various ATK % stats scale far better on him than a normal champ
3) He has a very high multiplier on his first attack - 2.5x for two attacks is very strong
4) Last, but not least, he has both an AOE stun AND an AOE turn meter reduction ability. The two of those combined make him almost unmatched for wave control in dungeons/campaign as well as for arena
How you plan on using him will depend on your other champs. Personally, I used him literally everywhere for a very long time, but especially, I used him as my campaign farmer for Brutal.
Kramaswamy, I Did Not know about the multiplier but I'm pairing the turn reduction With A Champion that attacks Often. Also I just did my first ancient shard since the one that I got Big'un from and I got Cupidus so LoL are they good for spider?
Big'un is very good for early spider - up until around Spider 13 he can essentially carry you through.
At later stages he's less useful, because the spiderlings are much harder to kill. He's not useless, but his usability drops significantly.
Can't comment on Cupidus as I don't have him, but I'd suggest you consult Raid Shadow Legends List of Champions by Ranking | AyumiLove - very useful resource.