With a regular IRA, you eventually have to pay tax

With a regular IRA, you eventually have to pay taxes on the money you withdraw. With a Roth IRA, you don't pay taxes on it. So when you convert a regular IRA to a Roth, you have to pay the taxes on the regular IRA to make up for the taxes you won't pay down the road. Of course, assuming your regular IRA will be worth a lot more down the road (like for instance you have almost all of it in CTIX), it's better to pay the tax now on the smaller amount so that you won't have to pay taxes down the road on the bigger amount.

If you have a regular IRA it's a good idea to convert it to a Roth, needless to say.