The X-Files didn't have a plan to take it into multiple seasons. They were flying by the seat of their pants and too many episodes were written with what appears to have been no oversight, leading to the same actors playing different versions of their characters.

You don't have to have the story blocked out into specific episodes, but with the type of series we're talking about (one with a serial element) you need to have some conception of an overall story.

How many show creators have said in interviews that they have the entire series carefully crafted out already in advance? A number of them, including Kring from Heroes as I recall. Even when they come up with an idea for a single season it's always gone to crap. Even for series such as 24 where the season's story is likely very well developed prior to shooting, they seem to have an easy time letting it go to crap.

Some writers and creators are better than others and can keep a series going as if it were all crafted out ahead of time. They take a look at past episodes and factor them in to future ones. Keeping a show cohesive and continuity in check is important and some people just don't have what it takes, even if only to manage this in the hands of others.