Dear everyone:
Is a tracker necessary?
After reading this article, I wonder is it necessary to build a tracker?
Does an omnidirectional antenna work well?
(http://www.amsat.org/xtra/Getting%20Started%203.pdf)
I am a beginner, and are there something interesting from satellite I can receive?
(http://www.amsat.org/?page_id=1869)
It seems that only simple “hello” message can be sent using satellite.
Today, if internet can communicate well, why do you still interesting in this?
This project is only an adult toy, or actually useful?
As far as I understand yes and no. Let me elaborate on that as far as I understand it. I guess that since I am no ham radio amateur I may get some stuff wrong here so please any hams out there feel free to correct me.
Omnidirectional antennas (like Eggbeaters) are nice and can be used as an alternative in certain scenarios but there are some limitations, one of the most significant is that you the target satellite signal must be almost over head to receive something useful. Using a Yagi-Uda (or a helix) combined with a tracker can change that allowing a single ground station to cover a much bigger area (this is also affected by the tracked satellites orbital characteristics too).
On interesting stuff for you to receive, well first and foremost many satellites that transmit telemetry data via amateur radio bands. This is important for satellite operators that want to be sure about the status of their satellites in orbit.Some weather satellites provide image data and there are several software packages able to decode such transmissions providing real-time weather imagery. And other data collected for various fields of science.
As is currently SatNOGS is specialized in receiving data in VHF and UHF bands. Personally I am interested on the project cause, mankind has dedicated some serious manpower, and resources to put thousands of satellites in orbit collecting data about our planet and near-earth space, opening access to even part of these data is really important of course we love creating new an interesting stuff, learning about orbital physics and RF communications
We dedicate lot’s of time and resources on this project and we believe it worths the effort 
spot on! eggbeaters are typically right-hand circularly polarized above the horizon. There are other omnis and they all have their pros and cons. I built a turnstile dual-moxon antenna for satellites and had horrible results (even with a preamp)… And that leads me to my next point - for picking up most low powered satellites with an omni a preamp would be a must. The closer the preamp is to the antenna itself the better. Some of the commercial mast-mounted preamp models go for hundreds of dollars (USD) per band.
That said, there are cases where the omni setup is a better idea (logistics being the likely reason) and with the right match of equipment can be a good ground station. I filed this a while back to facilitate such stations: https://github.com/satnogs/satnogs-client/issues/52
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