In the latest look at the underlying components of the S&P 500 ordered by largest market capitalization, Rockwell Automation, Inc. ( ROK) has taken over the #253 spot from SanDisk Corp. ( SNDK), according to The Online Investor.Market capitalization is an important data point for investors to keep an eye on, for various reasons. The most basic reason is that it gives a true comparison of the value attributed by the stock market to a given company's stock. Many beginning investors look at one stock trading at $10 and another trading at $20 and mistakenly think the latter company is worth twice as much — that of course is a completely meaningless comparison without knowing how many shares of each company exist. But comparing market capitalization (factoring in those share counts) creates a true "apples-to-apples" comparison of the value of two stocks. In the case of Rockwell Automation, Inc. ( ROK), the market cap is now $15.96 billion, versus SanDisk Corp. ( SNDK) at $15.72 billion. Below is a chart of Rockwell Automation, Inc. versus SanDisk Corp. plotting their respective size rank within the S&P 500 over time (ROK plotted in blue; SNDK plotted in green): Below is a three month price history chart comparing the stock performance of ROK vs. SNDK: Another reason market capitalization is important is where it places a company in terms of its size tier in relation to peers — much like the way a mid-size sedan is typically compared to other mid-size sedans (and not SUV's). This can have a direct impact on which mutual funds and ETFs are willing to own the stock. For instance, a mutual fund that is focused solely on Large Cap stocks may for example only be interested in those companies sized $10 billion or larger. Another illustrative example is the S&P MidCap index which essentially takes the S&P 500 index and "tosses out" the biggest 100 companies so as to focus solely on the 400 smaller "up-and-comers" (which in the right environment can outperform their larger rivals). So a company's market cap, especially in relation to other companies, carries great importance, and for this reason we at The Online Investor find value to putting together these rankings daily.