Using line selections allows you to move lines of code around much quicker. SlickEdit treats line selections differently than character selections. When pasting a line selection, it is pasted above or below the current line, regardless of the cursor location on that line. By contrast, character selections are pasted at the cursor.
The location where the line is pasted is based on the Line insert style setting on the More tab of the General Options dialog (Tools > Options > General). Note that this setting is also controlled by the def_line_insert configuration variable (set to A for After or B for Before).