I'm not saying you have to...because clearly you don't.
But, it is healthier for the fish to cycle the tank before putting them in there..they don't have to live through the cycle if you cycle it before they get put in there.
Otherwise they have to endure severe ammonia spikes as the tank eventually cycles (over about a months time) not to mention you have to change a great deal of the water as you go along..so it's more expensive on you (i.e. extra water everyday and water conditioner, plus time and effort).
People do fishy cycles all the time...but then they are surprised when their fish have weakened immune systems and die at earlier ages...it's from going through a cycle alot of time..from what their body had to go through over that months time frame. It can also just straight up kill some fish..the stress of it all is just too much. It's up to each individual owner to decide what's right for THEIR individual fish..what is said here is merely a suggestion and facts on what can happen later on.
I will say this...alot of people condem others for not doing a fishless cycle...but in some areas it is extremely hard to come up with the supplies to do a fishless cycle. I'm doing one right now (a 20g community tank at my house) and lemme tell you..I had to search HIGH and
low for the supplies, they aren't just readily available too much (at least not around here). It's hard to find pure straight up ammonia..most of it has other chemicals, perfumes and dyes in it...which aren't good.
You can use ornaments, gravel, and filter bags from other tanks (that's called seeding, if I'm not mistaken), but all that does is speed up the cycle a bit...it doesn't make your tank "ready cycled".
Cycling tanks is tricky, complicated business lol..that's why I like smaller individual tanks myself..so I don't have to worry with it (yes I know I'm lazy, but heck..can you blame me lol)
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**Edit**
Here is a great link to a fishless cycle I used...so far so good. (I knew I saw it her and I have it saved..but only in word format lol..finally found the link
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http
/www.fishforums.net/index.php?showtopic=90452&hl=