While you have an arrow, it's the same as it is a line, the value of the buffer at that time is the price where the arrow is. But arrows appear only here and there. What is going on in the other candles where there are no arrows? For these candles the value that the indicator gives you is EMPTY_VALUE, which is a predefined constant and contains the maximum integer value (2147483647). It's some huge value, that's all. MT4 knows how to react on this value. Some indicators may use 0 instead.
So, there are some blocks in a special category for indicators. But also Condition can do the job. In Condition, if one of the sides equals to EMPTY_VALUE, the block does not pass at all. So you can do this condition [Indicator > 0] and the block will only pass when there is an arrow. This is also what the "Indicator is visible" block does.
"Indicator appear" is also interesting, check it's description. Also in Condition, a crossover with some value like 1000 detects something - imagine that for the previous candle the indicator was 2147483647 and for the current candle it is 1.2345, which means that it crossed 1000 